diff --git a/composer.json b/composer.json
index 6588612e4..3289dc143 100644
--- a/composer.json
+++ b/composer.json
@@ -18,7 +18,7 @@
     "doctrine/coding-standard": "^6.0",
     "phpbench/phpbench": "^0.14",
     "phpstan/extension-installer": "^1.0",
-    "phpstan/phpstan": "0.12.11",
+    "phpstan/phpstan": "0.12.18",
     "phpstan/phpstan-phpunit": "0.12.6",
     "phpstan/phpstan-strict-rules": "0.12.2",
     "phpunit/phpunit": "^7.2|^8.5",
@@ -54,7 +54,7 @@
     "lint" : "phpcs",
     "fix" : "phpcbf",
     "stan": "phpstan analyse --ansi --memory-limit 2048M",
-    "phpstan-baseline": "phpstan analyse --ansi --error-format baselineNeon > phpstan-baseline.neon",
+    "phpstan-baseline": "phpstan analyse --ansi --generate-baseline=phpstan-baseline.neon",
     "check": "composer lint && composer stan && composer test"
   }
 }
diff --git a/phpstan-baseline.neon b/phpstan-baseline.neon
index 1be9308e7..8565ae099 100644
--- a/phpstan-baseline.neon
+++ b/phpstan-baseline.neon
@@ -50,11 +50,6 @@ parameters:
 			count: 1
 			path: src/Error/Error.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"\\?GraphQL\\\\Language\\\\SourceLocation\"\\.$#"
-			count: 1
-			path: src/Error/Error.php
-
 		-
 			message: "#^Only booleans are allowed in &&, GraphQL\\\\Language\\\\AST\\\\Location given on the left side\\.$#"
 			count: 1
@@ -160,11 +155,6 @@ parameters:
 			count: 4
 			path: src/Executor/ReferenceExecutor.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"\\?GraphQL\\\\Type\\\\Definition\\\\ObjectType\"\\.$#"
-			count: 1
-			path: src/Executor/ReferenceExecutor.php
-
 		-
 			message: "#^Only booleans are allowed in &&, GraphQL\\\\Language\\\\AST\\\\BooleanValueNode\\|GraphQL\\\\Language\\\\AST\\\\EnumValueNode\\|GraphQL\\\\Language\\\\AST\\\\FloatValueNode\\|GraphQL\\\\Language\\\\AST\\\\IntValueNode\\|GraphQL\\\\Language\\\\AST\\\\ListValueNode\\|GraphQL\\\\Language\\\\AST\\\\NullValueNode\\|GraphQL\\\\Language\\\\AST\\\\ObjectValueNode\\|GraphQL\\\\Language\\\\AST\\\\StringValueNode\\|GraphQL\\\\Language\\\\AST\\\\VariableNode\\|null given on the right side\\.$#"
 			count: 1
@@ -200,11 +190,6 @@ parameters:
 			count: 2
 			path: src/Experimental/Executor/CoroutineExecutor.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 3
-			path: src/Experimental/Executor/CoroutineExecutor.php
-
 		-
 			message: "#^Variable property access on object\\.$#"
 			count: 2
@@ -225,11 +210,6 @@ parameters:
 			count: 2
 			path: src/GraphQL.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 1
-			path: src/GraphQL.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 1
@@ -255,61 +235,11 @@ parameters:
 			count: 1
 			path: src/Language/Parser.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\AST\\\\VariableDefinitionNode\"\\.$#"
-			count: 1
-			path: src/Language/Parser.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\AST\\\\SelectionNode\"\\.$#"
-			count: 1
-			path: src/Language/Parser.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\AST\\\\ArgumentNode\"\\.$#"
-			count: 2
-			path: src/Language/Parser.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\AST\\\\OperationTypeDefinitionNode\"\\.$#"
-			count: 1
-			path: src/Language/Parser.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\AST\\\\FieldDefinitionNode\"\\.$#"
-			count: 1
-			path: src/Language/Parser.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\AST\\\\InputValueDefinitionNode\"\\.$#"
-			count: 2
-			path: src/Language/Parser.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\AST\\\\EnumValueDefinitionNode\"\\.$#"
-			count: 1
-			path: src/Language/Parser.php
-
 		-
 			message: "#^Short ternary operator is not allowed\\. Use null coalesce operator if applicable or consider using long ternary\\.$#"
 			count: 1
 			path: src/Language/Printer.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"string\"\\.$#"
-			count: 18
-			path: src/Language/Printer.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\AST\\\\NameNode\"\\.$#"
-			count: 1
-			path: src/Language/Printer.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"bool\"\\.$#"
-			count: 3
-			path: src/Language/Printer.php
-
 		-
 			message: "#^Only booleans are allowed in a ternary operator condition, array\\<GraphQL\\\\Language\\\\AST\\\\NamedTypeNode\\>\\|null given\\.$#"
 			count: 2
@@ -395,21 +325,11 @@ parameters:
 			count: 5
 			path: src/Server/Helper.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Error\\\\Error\"\\.$#"
-			count: 1
-			path: src/Server/Helper.php
-
 		-
 			message: "#^Only booleans are allowed in a ternary operator condition, string given\\.$#"
 			count: 1
 			path: src/Server/Helper.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Executor\\\\ExecutionResult\"\\.$#"
-			count: 1
-			path: src/Server/Helper.php
-
 		-
 			message: "#^Only booleans are allowed in an if condition, \\(callable\\)\\|null given\\.$#"
 			count: 1
@@ -420,11 +340,6 @@ parameters:
 			count: 1
 			path: src/Server/Helper.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 2
-			path: src/Server/Helper.php
-
 		-
 			message: "#^Short ternary operator is not allowed\\. Use null coalesce operator if applicable or consider using long ternary\\.$#"
 			count: 2
@@ -470,11 +385,6 @@ parameters:
 			count: 1
 			path: src/Type/Definition/ObjectType.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"bool\"\\.$#"
-			count: 2
-			path: src/Type/Definition/QueryPlan.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, GraphQL\\\\Language\\\\AST\\\\SelectionSetNode\\|null given\\.$#"
 			count: 1
@@ -495,61 +405,21 @@ parameters:
 			count: 1
 			path: src/Type/Definition/Type.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 3
-			path: src/Type/Introspection.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"\\?GraphQL\\\\Type\\\\Definition\\\\ObjectType\"\\.$#"
-			count: 2
-			path: src/Type/Introspection.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 3
 			path: src/Type/Introspection.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"bool\"\\.$#"
-			count: 4
-			path: src/Type/Introspection.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, string\\|null given\\.$#"
 			count: 2
 			path: src/Type/Introspection.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"\\?array\"\\.$#"
-			count: 3
-			path: src/Type/Introspection.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"\\?GraphQL\\\\Type\\\\Definition\\\\Type\"\\.$#"
-			count: 2
-			path: src/Type/Introspection.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"string\"\\.$#"
-			count: 3
-			path: src/Type/Introspection.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"\\?string\"\\.$#"
-			count: 4
-			path: src/Type/Introspection.php
-
 		-
 			message: "#^Short ternary operator is not allowed\\. Use null coalesce operator if applicable or consider using long ternary\\.$#"
 			count: 1
 			path: src/Type/Introspection.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Type\\\\Schema\"\\.$#"
-			count: 1
-			path: src/Type/Introspection.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, array\\<GraphQL\\\\Type\\\\Definition\\\\Type\\>\\|\\(callable\\) given\\.$#"
 			count: 1
@@ -610,21 +480,11 @@ parameters:
 			count: 1
 			path: src/Type/SchemaValidationContext.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"\\?GraphQL\\\\Language\\\\AST\\\\DirectiveDefinitionNode\"\\.$#"
-			count: 1
-			path: src/Type/SchemaValidationContext.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, GraphQL\\\\Error\\\\Error\\|null given\\.$#"
 			count: 1
 			path: src/Type/SchemaValidationContext.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"bool\"\\.$#"
-			count: 6
-			path: src/Type/SchemaValidationContext.php
-
 		-
 			message: "#^Only booleans are allowed in a ternary operator condition, GraphQL\\\\Language\\\\AST\\\\InputValueDefinitionNode given\\.$#"
 			count: 1
@@ -785,21 +645,11 @@ parameters:
 			count: 1
 			path: src/Utils/ASTDefinitionBuilder.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Type\\\\Definition\\\\Type\"\\.$#"
-			count: 2
-			path: src/Utils/ASTDefinitionBuilder.php
-
 		-
 			message: "#^Only booleans are allowed in a ternary operator condition, GraphQL\\\\Language\\\\AST\\\\NodeList\\<GraphQL\\\\Language\\\\AST\\\\EnumValueDefinitionNode\\>\\|null given\\.$#"
 			count: 1
 			path: src/Utils/ASTDefinitionBuilder.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 1
-			path: src/Utils/ASTDefinitionBuilder.php
-
 		-
 			message: "#^Only booleans are allowed in a ternary operator condition, array\\<GraphQL\\\\Language\\\\AST\\\\NamedTypeNode\\>\\|null given\\.$#"
 			count: 1
@@ -810,11 +660,6 @@ parameters:
 			count: 1
 			path: src/Utils/ASTDefinitionBuilder.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"bool\"\\.$#"
-			count: 2
-			path: src/Utils/BreakingChangesFinder.php
-
 		-
 			message: "#^Short ternary operator is not allowed\\. Use null coalesce operator if applicable or consider using long ternary\\.$#"
 			count: 1
@@ -825,41 +670,11 @@ parameters:
 			count: 3
 			path: src/Utils/BuildSchema.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: src/Utils/BuildSchema.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Type\\\\Definition\\\\Type\"\\.$#"
-			count: 1
-			path: src/Utils/BuildSchema.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 1
-			path: src/Utils/BuildSchema.php
-
 		-
 			message: "#^Only booleans are allowed in &&, array\\<bool\\>\\|null given on the left side\\.$#"
 			count: 1
 			path: src/Utils/PairSet.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 5
-			path: src/Utils/SchemaExtender.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"string\"\\.$#"
-			count: 1
-			path: src/Utils/SchemaExtender.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Type\\\\Definition\\\\Directive\"\\.$#"
-			count: 1
-			path: src/Utils/SchemaExtender.php
-
 		-
 			message: "#^Only booleans are allowed in an if condition, GraphQL\\\\Type\\\\Definition\\\\Type\\|null given\\.$#"
 			count: 1
@@ -875,21 +690,6 @@ parameters:
 			count: 1
 			path: src/Utils/SchemaExtender.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Type\\\\Definition\\\\Type\"\\.$#"
-			count: 1
-			path: src/Utils/SchemaExtender.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"bool\"\\.$#"
-			count: 3
-			path: src/Utils/SchemaPrinter.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"string\"\\.$#"
-			count: 7
-			path: src/Utils/SchemaPrinter.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 2
@@ -935,11 +735,6 @@ parameters:
 			count: 1
 			path: src/Utils/TypeInfo.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"bool\"\\.$#"
-			count: 1
-			path: src/Utils/TypeInfo.php
-
 		-
 			message: "#^Only booleans are allowed in &&, GraphQL\\\\Type\\\\Definition\\\\FieldArgument\\|null given on the left side\\.$#"
 			count: 1
@@ -975,26 +770,11 @@ parameters:
 			count: 1
 			path: src/Utils/Utils.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: src/Utils/Utils.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"string\"\\.$#"
-			count: 2
-			path: src/Utils/Utils.php
-
 		-
 			message: "#^Only booleans are allowed in an if condition, GraphQL\\\\Type\\\\Definition\\\\EnumValueDefinition\\|null given\\.$#"
 			count: 1
 			path: src/Utils/Value.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"string\"\\.$#"
-			count: 1
-			path: src/Utils/Value.php
-
 		-
 			message: "#^Only booleans are allowed in a ternary operator condition, array\\<string\\> given\\.$#"
 			count: 2
@@ -1020,21 +800,6 @@ parameters:
 			count: 1
 			path: src/Validator/DocumentValidator.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"bool\"\\.$#"
-			count: 1
-			path: src/Validator/DocumentValidator.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\VisitorOperation\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/ExecutableDefinitions.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/FieldsOnCorrectType.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, \\(GraphQL\\\\Type\\\\Definition\\\\CompositeType&GraphQL\\\\Type\\\\Definition\\\\Type\\)\\|null given\\.$#"
 			count: 1
@@ -1055,11 +820,6 @@ parameters:
 			count: 1
 			path: src/Validator/Rules/FieldsOnCorrectType.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 2
-			path: src/Validator/Rules/FragmentsOnCompositeTypes.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, GraphQL\\\\Language\\\\AST\\\\NamedTypeNode given\\.$#"
 			count: 1
@@ -1070,21 +830,11 @@ parameters:
 			count: 2
 			path: src/Validator/Rules/FragmentsOnCompositeTypes.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"string\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/KnownArgumentNames.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 2
 			path: src/Validator/Rules/KnownArgumentNames.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/KnownArgumentNamesOnDirectives.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, array\\<string\\>\\|null given\\.$#"
 			count: 1
@@ -1105,56 +855,21 @@ parameters:
 			count: 1
 			path: src/Validator/Rules/KnownDirectives.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/KnownFragmentNames.php
-
 		-
 			message: "#^Only booleans are allowed in an if condition, GraphQL\\\\Language\\\\AST\\\\FragmentDefinitionNode\\|null given\\.$#"
 			count: 1
 			path: src/Validator/Rules/KnownFragmentNames.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\VisitorOperation\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/KnownTypeNames.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/KnownTypeNames.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 1
 			path: src/Validator/Rules/KnownTypeNames.php
 
-		-
-			message: "#^Anonymous function sometimes return something but return statement at the end is missing\\.$#"
-			count: 1
-			path: src/Validator/Rules/LoneAnonymousOperation.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"bool\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/LoneAnonymousOperation.php
-
 		-
 			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
 			count: 1
 			path: src/Validator/Rules/LoneAnonymousOperation.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/LoneSchemaDefinition.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\VisitorOperation\"\\.$#"
-			count: 2
-			path: src/Validator/Rules/NoFragmentCycles.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 3
@@ -1165,46 +880,21 @@ parameters:
 			count: 1
 			path: src/Validator/Rules/NoFragmentCycles.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 3
-			path: src/Validator/Rules/NoUndefinedVariables.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 1
 			path: src/Validator/Rules/NoUndefinedVariables.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\VisitorOperation\"\\.$#"
-			count: 2
-			path: src/Validator/Rules/NoUnusedFragments.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/NoUnusedFragments.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 1
 			path: src/Validator/Rules/NoUnusedFragments.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 3
-			path: src/Validator/Rules/NoUnusedVariables.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 1
 			path: src/Validator/Rules/NoUnusedVariables.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/OverlappingFieldsCanBeMerged.php
-
 		-
 			message: "#^Only booleans are allowed in a ternary operator condition, GraphQL\\\\Language\\\\AST\\\\NameNode\\|null given\\.$#"
 			count: 1
@@ -1240,21 +930,6 @@ parameters:
 			count: 3
 			path: src/Validator/Rules/OverlappingFieldsCanBeMerged.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 2
-			path: src/Validator/Rules/OverlappingFieldsCanBeMerged.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"string\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/OverlappingFieldsCanBeMerged.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 2
-			path: src/Validator/Rules/PossibleFragmentSpreads.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, \\(GraphQL\\\\Type\\\\Definition\\\\CompositeType&GraphQL\\\\Type\\\\Definition\\\\Type\\)\\|null given\\.$#"
 			count: 1
@@ -1265,11 +940,6 @@ parameters:
 			count: 1
 			path: src/Validator/Rules/PossibleFragmentSpreads.php
 
-		-
-			message: "#^Anonymous function sometimes return something but return statement at the end is missing\\.$#"
-			count: 2
-			path: src/Validator/Rules/ProvidedRequiredArguments.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, GraphQL\\\\Type\\\\Definition\\\\FieldDefinition given\\.$#"
 			count: 1
@@ -1300,11 +970,6 @@ parameters:
 			count: 1
 			path: src/Validator/Rules/ProvidedRequiredArgumentsOnDirectives.php
 
-		-
-			message: "#^Anonymous function sometimes return something but return statement at the end is missing\\.$#"
-			count: 1
-			path: src/Validator/Rules/ProvidedRequiredArgumentsOnDirectives.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, array\\|null given\\.$#"
 			count: 1
@@ -1315,16 +980,6 @@ parameters:
 			count: 1
 			path: src/Validator/Rules/ProvidedRequiredArgumentsOnDirectives.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 2
-			path: src/Validator/Rules/QueryComplexity.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\VisitorOperation\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/QueryComplexity.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 3
@@ -1335,11 +990,6 @@ parameters:
 			count: 1
 			path: src/Validator/Rules/QueryComplexity.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/QueryDepth.php
-
 		-
 			message: "#^Short ternary operator is not allowed\\. Use null coalesce operator if applicable or consider using long ternary\\.$#"
 			count: 2
@@ -1365,11 +1015,6 @@ parameters:
 			count: 1
 			path: src/Validator/Rules/QuerySecurityRule.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/ScalarLeafs.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, GraphQL\\\\Type\\\\Definition\\\\OutputType\\|null given\\.$#"
 			count: 1
@@ -1385,66 +1030,26 @@ parameters:
 			count: 1
 			path: src/Validator/Rules/ScalarLeafs.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 2
-			path: src/Validator/Rules/UniqueArgumentNames.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\VisitorOperation\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/UniqueArgumentNames.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 1
 			path: src/Validator/Rules/UniqueArgumentNames.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/UniqueDirectivesPerLocation.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\VisitorOperation\"\\.$#"
-			count: 2
-			path: src/Validator/Rules/UniqueFragmentNames.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 1
 			path: src/Validator/Rules/UniqueFragmentNames.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 2
-			path: src/Validator/Rules/UniqueInputFieldNames.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\VisitorOperation\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/UniqueInputFieldNames.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 1
 			path: src/Validator/Rules/UniqueInputFieldNames.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Language\\\\VisitorOperation\"\\.$#"
-			count: 2
-			path: src/Validator/Rules/UniqueOperationNames.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 1
 			path: src/Validator/Rules/UniqueOperationNames.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 2
-			path: src/Validator/Rules/UniqueVariableNames.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 1
@@ -1455,21 +1060,6 @@ parameters:
 			count: 1
 			path: src/Validator/Rules/ValidationRule.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 8
-			path: src/Validator/Rules/ValuesOfCorrectType.php
-
-		-
-			message: "#^Anonymous function sometimes return something but return statement at the end is missing\\.$#"
-			count: 1
-			path: src/Validator/Rules/ValuesOfCorrectType.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"string\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/ValuesOfCorrectType.php
-
 		-
 			message: "#^Only booleans are allowed in \\|\\|, GraphQL\\\\Type\\\\Definition\\\\EnumType\\|GraphQL\\\\Type\\\\Definition\\\\InputObjectType\\|GraphQL\\\\Type\\\\Definition\\\\ListOfType\\|GraphQL\\\\Type\\\\Definition\\\\NonNull\\|GraphQL\\\\Type\\\\Definition\\\\ScalarType given on the left side\\.$#"
 			count: 1
@@ -1490,21 +1080,11 @@ parameters:
 			count: 1
 			path: src/Validator/Rules/ValuesOfCorrectType.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: src/Validator/Rules/VariablesAreInputTypes.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, GraphQL\\\\Type\\\\Definition\\\\Type\\|null given\\.$#"
 			count: 1
 			path: src/Validator/Rules/VariablesAreInputTypes.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 3
-			path: src/Validator/Rules/VariablesInAllowedPosition.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, GraphQL\\\\Type\\\\Definition\\\\Type\\|null given\\.$#"
 			count: 1
@@ -1515,16 +1095,6 @@ parameters:
 			count: 1
 			path: src/Validator/Rules/VariablesInAllowedPosition.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"bool\"\\.$#"
-			count: 1
-			path: src/Validator/ValidationContext.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: src/Validator/ValidationContext.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 3
@@ -2032,11 +1602,6 @@ parameters:
 
 		-
 			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 5
-			path: tests/Type/QueryPlanTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"bool\"\\.$#"
 			count: 1
 			path: tests/Type/QueryPlanTest.php
 
@@ -2045,158 +1610,23 @@ parameters:
 			count: 1
 			path: tests/Type/QueryPlanTest.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 2
-			path: tests/Type/ResolveInfoTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"string\"\\.$#"
-			count: 1
-			path: tests/Type/SchemaTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 1
-			path: tests/Type/SchemaTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 3
-			path: tests/Type/StandardTypesTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 5
-			path: tests/Type/TypeLoaderTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 5
-			path: tests/Type/TypeLoaderTest.php
-
 		-
 			message: "#^Variable property access on \\$this\\(GraphQL\\\\Tests\\\\Type\\\\TypeLoaderTest\\)\\.$#"
 			count: 1
 			path: tests/Type/TypeLoaderTest.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"stdClass\"\\.$#"
-			count: 1
-			path: tests/Type/TypeLoaderTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Type\\\\Definition\\\\InterfaceType\"\\.$#"
-			count: 1
-			path: tests/Type/TypeLoaderTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 3
-			path: tests/Type/ValidationTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 5
-			path: tests/Type/ValidationTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Type\\\\Definition\\\\ListOfType\"\\.$#"
-			count: 1
-			path: tests/Type/ValidationTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Type\\\\Definition\\\\NonNull\"\\.$#"
-			count: 2
-			path: tests/Type/ValidationTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Type\\\\Definition\\\\ObjectType\"\\.$#"
-			count: 1
-			path: tests/Type/ValidationTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Type\\\\Definition\\\\EnumType\"\\.$#"
-			count: 1
-			path: tests/Type/ValidationTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Type\\\\Definition\\\\InputObjectType\"\\.$#"
-			count: 1
-			path: tests/Type/ValidationTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Type\\\\Definition\\\\UnionType\"\\.$#"
-			count: 1
-			path: tests/Type/ValidationTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"GraphQL\\\\Type\\\\Definition\\\\InterfaceType\"\\.$#"
-			count: 1
-			path: tests/Type/ValidationTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"\\?GraphQL\\\\Type\\\\Definition\\\\ObjectType\"\\.$#"
-			count: 1
-			path: tests/Type/ValidationTest.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, stdClass given\\.$#"
 			count: 1
 			path: tests/Utils/AstFromValueTest.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 7
-			path: tests/Utils/ExtractTypesTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 2
-			path: tests/Utils/MixedStoreTest.php
-
 		-
 			message: "#^Construct empty\\(\\) is not allowed\\. Use more strict comparison\\.$#"
 			count: 2
 			path: tests/Utils/MixedStoreTest.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 5
-			path: tests/Utils/SchemaExtenderTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"string\"\\.$#"
-			count: 5
-			path: tests/Utils/SchemaExtenderTest.php
-
 		-
 			message: "#^Short ternary operator is not allowed\\. Use null coalesce operator if applicable or consider using long ternary\\.$#"
 			count: 1
 			path: tests/Utils/ValueFromAstTest.php
 
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 3
-			path: tests/Validator/OverlappingFieldsCanBeMergedTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 1
-			path: tests/Validator/QueryComplexityTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 1
-			path: tests/Validator/QueryComplexityTest.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"array\"\\.$#"
-			count: 3
-			path: tests/Validator/ValidatorTestCase.php
-
-		-
-			message: "#^Anonymous function should have native return typehint \"void\"\\.$#"
-			count: 2
-			path: tests/Validator/ValidatorTestCase.php
-
diff --git a/src/Error/Error.php b/src/Error/Error.php
index dfa98715e..7e6056fc0 100644
--- a/src/Error/Error.php
+++ b/src/Error/Error.php
@@ -278,7 +278,7 @@ static function ($pos) use ($source) : SourceLocation {
             } elseif ($nodes) {
                 $locations       = array_filter(
                     array_map(
-                        static function ($node) {
+                        static function ($node) : ?SourceLocation {
                             if ($node->loc && $node->loc->source) {
                                 return $node->loc->source->getLocation($node->loc->start);
                             }
diff --git a/src/Executor/ReferenceExecutor.php b/src/Executor/ReferenceExecutor.php
index 7093fdd58..afcb59434 100644
--- a/src/Executor/ReferenceExecutor.php
+++ b/src/Executor/ReferenceExecutor.php
@@ -1038,7 +1038,7 @@ private function defaultTypeResolver($value, $contextValue, ResolveInfo $info, A
         }
         if (! empty($promisedIsTypeOfResults)) {
             return $this->exeContext->promiseAdapter->all($promisedIsTypeOfResults)
-                ->then(static function ($isTypeOfResults) use ($possibleTypes) {
+                ->then(static function ($isTypeOfResults) use ($possibleTypes) : ?ObjectType {
                     foreach ($isTypeOfResults as $index => $result) {
                         if ($result) {
                             return $possibleTypes[$index];
diff --git a/src/Experimental/Executor/CoroutineExecutor.php b/src/Experimental/Executor/CoroutineExecutor.php
index 185f54a74..5a073eb8e 100644
--- a/src/Experimental/Executor/CoroutineExecutor.php
+++ b/src/Experimental/Executor/CoroutineExecutor.php
@@ -226,7 +226,7 @@ public function doExecute() : Promise
         $this->run();
 
         if ($this->pending > 0) {
-            return $this->promiseAdapter->create(function (callable $resolve) {
+            return $this->promiseAdapter->create(function (callable $resolve) : void {
                 $this->doResolve = $resolve;
             });
         }
@@ -313,13 +313,13 @@ private function run()
                         $this->promiseAdapter
                             ->then(
                                 $value,
-                                function ($value) use ($strand) {
+                                function ($value) use ($strand) : void {
                                     $strand->success = true;
                                     $strand->value   = $value;
                                     $this->queue->enqueue($strand);
                                     $this->done();
                                 },
-                                function (Throwable $throwable) use ($strand) {
+                                function (Throwable $throwable) use ($strand) : void {
                                     $strand->success = false;
                                     $strand->value   = $throwable;
                                     $this->queue->enqueue($strand);
diff --git a/src/GraphQL.php b/src/GraphQL.php
index ef5d7b4be..3b653b6ab 100644
--- a/src/GraphQL.php
+++ b/src/GraphQL.php
@@ -213,7 +213,7 @@ public static function execute(
         if ($promiseAdapter instanceof SyncPromiseAdapter) {
             $result = $promiseAdapter->wait($result)->toArray();
         } else {
-            $result = $result->then(static function (ExecutionResult $r) {
+            $result = $result->then(static function (ExecutionResult $r) : array {
                 return $r->toArray();
             });
         }
diff --git a/src/Language/AST/IntValueNode.php b/src/Language/AST/IntValueNode.php
index 3441b7bc2..0bdab5be0 100644
--- a/src/Language/AST/IntValueNode.php
+++ b/src/Language/AST/IntValueNode.php
@@ -9,6 +9,6 @@ class IntValueNode extends Node implements ValueNode
     /** @var string */
     public $kind = NodeKind::INT;
 
-    /** @var string */
+    /** @var mixed */
     public $value;
 }
diff --git a/src/Language/AST/NamedTypeNode.php b/src/Language/AST/NamedTypeNode.php
index 7a44b2f67..da62aeddf 100644
--- a/src/Language/AST/NamedTypeNode.php
+++ b/src/Language/AST/NamedTypeNode.php
@@ -9,6 +9,6 @@ class NamedTypeNode extends Node implements TypeNode
     /** @var string */
     public $kind = NodeKind::NAMED_TYPE;
 
-    /** @var NameNode */
+    /** @var NameNode|string */
     public $name;
 }
diff --git a/src/Language/AST/NodeList.php b/src/Language/AST/NodeList.php
index 62a776b24..be77b0183 100644
--- a/src/Language/AST/NodeList.php
+++ b/src/Language/AST/NodeList.php
@@ -83,8 +83,8 @@ public function offsetGet($offset)// : Node
     }
 
     /**
-     * @param int|string   $offset
-     * @param Node|mixed[] $value
+     * @param int|string|null $offset
+     * @param Node|mixed[]    $value
      *
      * @phpstan-param T|mixed[] $value
      */
diff --git a/src/Language/Parser.php b/src/Language/Parser.php
index b0ccc7b7c..0328fbdb3 100644
--- a/src/Language/Parser.php
+++ b/src/Language/Parser.php
@@ -550,7 +550,7 @@ private function parseVariableDefinitions() : NodeList
         return $this->peek(Token::PAREN_L)
             ? $this->many(
                 Token::PAREN_L,
-                function () {
+                function () : VariableDefinitionNode {
                     return $this->parseVariableDefinition();
                 },
                 Token::PAREN_R
@@ -601,7 +601,7 @@ private function parseSelectionSet() : SelectionSetNode
             [
                 'selections' => $this->many(
                     Token::BRACE_L,
-                    function () {
+                    function () : SelectionNode {
                         return $this->parseSelection();
                     },
                     Token::BRACE_R
@@ -656,10 +656,10 @@ private function parseField() : FieldNode
     private function parseArguments(bool $isConst) : NodeList
     {
         $parseFn = $isConst
-            ? function () {
+            ? function () : ArgumentNode {
                 return $this->parseConstArgument();
             }
-            : function () {
+            : function () : ArgumentNode {
                 return $this->parseArgument();
             };
 
@@ -1090,7 +1090,7 @@ private function parseSchemaDefinition() : SchemaDefinitionNode
 
         $operationTypes = $this->many(
             Token::BRACE_L,
-            function () {
+            function () : OperationTypeDefinitionNode {
                 return $this->parseOperationTypeDefinition();
             },
             Token::BRACE_R
@@ -1207,7 +1207,7 @@ private function parseFieldsDefinition() : NodeList
             $nodeList = $this->peek(Token::BRACE_L)
                 ? $this->many(
                     Token::BRACE_L,
-                    function () {
+                    function () : FieldDefinitionNode {
                         return $this->parseFieldDefinition();
                     },
                     Token::BRACE_R
@@ -1250,7 +1250,7 @@ private function parseArgumentsDefinition() : NodeList
         $nodeList = $this->peek(Token::PAREN_L)
             ? $this->many(
                 Token::PAREN_L,
-                function () {
+                function () : InputValueDefinitionNode {
                     return $this->parseInputValueDefinition();
                 },
                 Token::PAREN_R
@@ -1382,7 +1382,7 @@ private function parseEnumValuesDefinition() : NodeList
         $nodeList = $this->peek(Token::BRACE_L)
             ? $this->many(
                 Token::BRACE_L,
-                function () {
+                function () : EnumValueDefinitionNode {
                     return $this->parseEnumValueDefinition();
                 },
                 Token::BRACE_R
@@ -1440,7 +1440,7 @@ private function parseInputFieldsDefinition() : NodeList
         $nodeList = $this->peek(Token::BRACE_L)
             ? $this->many(
                 Token::BRACE_L,
-                function () {
+                function () : InputValueDefinitionNode {
                     return $this->parseInputValueDefinition();
                 },
                 Token::BRACE_R
diff --git a/src/Language/Printer.php b/src/Language/Printer.php
index 54b526d55..8e46ac329 100644
--- a/src/Language/Printer.php
+++ b/src/Language/Printer.php
@@ -99,15 +99,15 @@ public function printAST($ast)
             $ast,
             [
                 'leave' => [
-                    NodeKind::NAME => static function (NameNode $node) {
+                    NodeKind::NAME => static function (NameNode $node) : string {
                         return '' . $node->value;
                     },
 
-                    NodeKind::VARIABLE => static function (VariableNode $node) {
+                    NodeKind::VARIABLE => static function (VariableNode $node) : string {
                         return '$' . $node->name;
                     },
 
-                    NodeKind::DOCUMENT => function (DocumentNode $node) {
+                    NodeKind::DOCUMENT => function (DocumentNode $node) : string {
                         return $this->join($node->definitions, "\n\n") . "\n";
                     },
 
@@ -125,7 +125,7 @@ public function printAST($ast)
                             : $this->join([$op, $this->join([$name, $varDefs]), $directives, $selectionSet], ' ');
                     },
 
-                    NodeKind::VARIABLE_DEFINITION => function (VariableDefinitionNode $node) {
+                    NodeKind::VARIABLE_DEFINITION => function (VariableDefinitionNode $node) : string {
                         return $node->variable
                             . ': '
                             . $node->type
@@ -152,11 +152,11 @@ public function printAST($ast)
                         );
                     },
 
-                    NodeKind::ARGUMENT => static function (ArgumentNode $node) {
+                    NodeKind::ARGUMENT => static function (ArgumentNode $node) : string {
                         return $node->name . ': ' . $node->value;
                     },
 
-                    NodeKind::FRAGMENT_SPREAD => function (FragmentSpreadNode $node) {
+                    NodeKind::FRAGMENT_SPREAD => function (FragmentSpreadNode $node) : string {
                         return '...' . $node->name . $this->wrap(' ', $this->join($node->directives, ' '));
                     },
 
@@ -172,7 +172,7 @@ public function printAST($ast)
                         );
                     },
 
-                    NodeKind::FRAGMENT_DEFINITION => function (FragmentDefinitionNode $node) {
+                    NodeKind::FRAGMENT_DEFINITION => function (FragmentDefinitionNode $node) : string {
                         // Note: fragment variable definitions are experimental and may be changed or removed in the future.
                         return sprintf('fragment %s', $node->name)
                             . $this->wrap('(', $this->join($node->variableDefinitions, ', '), ')')
@@ -185,7 +185,7 @@ public function printAST($ast)
                         return $node->value;
                     },
 
-                    NodeKind::FLOAT => static function (FloatValueNode $node) {
+                    NodeKind::FLOAT => static function (FloatValueNode $node) : string {
                         return $node->value;
                     },
 
@@ -201,39 +201,39 @@ public function printAST($ast)
                         return $node->value ? 'true' : 'false';
                     },
 
-                    NodeKind::NULL => static function (NullValueNode $node) {
+                    NodeKind::NULL => static function (NullValueNode $node) : string {
                         return 'null';
                     },
 
-                    NodeKind::ENUM => static function (EnumValueNode $node) {
+                    NodeKind::ENUM => static function (EnumValueNode $node) : string {
                         return $node->value;
                     },
 
-                    NodeKind::LST => function (ListValueNode $node) {
+                    NodeKind::LST => function (ListValueNode $node) : string {
                         return '[' . $this->join($node->values, ', ') . ']';
                     },
 
-                    NodeKind::OBJECT => function (ObjectValueNode $node) {
+                    NodeKind::OBJECT => function (ObjectValueNode $node) : string {
                         return '{' . $this->join($node->fields, ', ') . '}';
                     },
 
-                    NodeKind::OBJECT_FIELD => static function (ObjectFieldNode $node) {
+                    NodeKind::OBJECT_FIELD => static function (ObjectFieldNode $node) : string {
                         return $node->name . ': ' . $node->value;
                     },
 
-                    NodeKind::DIRECTIVE => function (DirectiveNode $node) {
+                    NodeKind::DIRECTIVE => function (DirectiveNode $node) : string {
                         return '@' . $node->name . $this->wrap('(', $this->join($node->arguments, ', '), ')');
                     },
 
-                    NodeKind::NAMED_TYPE => static function (NamedTypeNode $node) {
+                    NodeKind::NAMED_TYPE => static function (NamedTypeNode $node) : string {
                         return $node->name;
                     },
 
-                    NodeKind::LIST_TYPE => static function (ListTypeNode $node) {
+                    NodeKind::LIST_TYPE => static function (ListTypeNode $node) : string {
                         return '[' . $node->type . ']';
                     },
 
-                    NodeKind::NON_NULL_TYPE => static function (NonNullTypeNode $node) {
+                    NodeKind::NON_NULL_TYPE => static function (NonNullTypeNode $node) : string {
                         return $node->type . '!';
                     },
 
@@ -248,7 +248,7 @@ public function printAST($ast)
                         );
                     },
 
-                    NodeKind::OPERATION_TYPE_DEFINITION => static function (OperationTypeDefinitionNode $def) {
+                    NodeKind::OPERATION_TYPE_DEFINITION => static function (OperationTypeDefinitionNode $def) : string {
                         return $def->operation . ': ' . $def->type;
                     },
 
@@ -270,7 +270,7 @@ public function printAST($ast)
                     }),
 
                     NodeKind::FIELD_DEFINITION => $this->addDescription(function (FieldDefinitionNode $def) {
-                        $noIndent = Utils::every($def->arguments, static function (string $arg) {
+                        $noIndent = Utils::every($def->arguments, static function (string $arg) : bool {
                             return strpos($arg, "\n") === false;
                         });
 
@@ -437,7 +437,7 @@ function (InterfaceTypeDefinitionNode $def) {
                     },
 
                     NodeKind::DIRECTIVE_DEFINITION => $this->addDescription(function (DirectiveDefinitionNode $def) {
-                        $noIndent = Utils::every($def->arguments, static function (string $arg) {
+                        $noIndent = Utils::every($def->arguments, static function (string $arg) : bool {
                             return strpos($arg, "\n") === false;
                         });
 
@@ -502,7 +502,7 @@ public function join($maybeArray, $separator = '')
                 $separator,
                 Utils::filter(
                     $maybeArray,
-                    static function ($x) {
+                    static function ($x) : bool {
                         return (bool) $x;
                     }
                 )
diff --git a/src/Server/Helper.php b/src/Server/Helper.php
index 396589ae2..ce9bf1c5c 100644
--- a/src/Server/Helper.php
+++ b/src/Server/Helper.php
@@ -266,7 +266,7 @@ private function promiseToExecuteOperation(
             if (! empty($errors)) {
                 $errors = Utils::map(
                     $errors,
-                    static function (RequestError $err) {
+                    static function (RequestError $err) : Error {
                         return Error::createLocatedError($err, null, null);
                     }
                 );
@@ -315,7 +315,7 @@ static function (RequestError $err) {
             );
         }
 
-        $applyErrorHandling = static function (ExecutionResult $result) use ($config) {
+        $applyErrorHandling = static function (ExecutionResult $result) use ($config) : ExecutionResult {
             if ($config->getErrorsHandler()) {
                 $result->setErrorsHandler($config->getErrorsHandler());
             }
@@ -434,7 +434,7 @@ private function resolveContextValue(
     public function sendResponse($result, $exitWhenDone = false)
     {
         if ($result instanceof Promise) {
-            $result->then(function ($actualResult) use ($exitWhenDone) {
+            $result->then(function ($actualResult) use ($exitWhenDone) : void {
                 $this->doSendResponse($actualResult, $exitWhenDone);
             });
         } else {
@@ -482,7 +482,7 @@ private function resolveHttpStatus($result)
         if (is_array($result) && isset($result[0])) {
             Utils::each(
                 $result,
-                static function ($executionResult, $index) {
+                static function ($executionResult, $index) : void {
                     if (! $executionResult instanceof ExecutionResult) {
                         throw new InvariantViolation(sprintf(
                             'Expecting every entry of batched query result to be instance of %s but entry at position %d is %s',
diff --git a/src/Type/Definition/QueryPlan.php b/src/Type/Definition/QueryPlan.php
index 03788232b..ace572fcb 100644
--- a/src/Type/Definition/QueryPlan.php
+++ b/src/Type/Definition/QueryPlan.php
@@ -79,7 +79,7 @@ public function getReferencedTypes() : array
 
     public function hasType(string $type) : bool
     {
-        return count(array_filter($this->getReferencedTypes(), static function (string $referencedType) use ($type) {
+        return count(array_filter($this->getReferencedTypes(), static function (string $referencedType) use ($type) : bool {
                 return $type === $referencedType;
         })) > 0;
     }
@@ -94,7 +94,7 @@ public function getReferencedFields() : array
 
     public function hasField(string $field) : bool
     {
-        return count(array_filter($this->getReferencedFields(), static function (string $referencedField) use ($field) {
+        return count(array_filter($this->getReferencedFields(), static function (string $referencedField) use ($field) : bool {
             return $field === $referencedField;
         })) > 0;
     }
diff --git a/src/Type/Introspection.php b/src/Type/Introspection.php
index 5bd928b31..fab7ecd0c 100644
--- a/src/Type/Introspection.php
+++ b/src/Type/Introspection.php
@@ -228,14 +228,14 @@ public static function _schema()
                     'types'            => [
                         'description' => 'A list of all types supported by this server.',
                         'type'        => new NonNull(new ListOfType(new NonNull(self::_type()))),
-                        'resolve'     => static function (Schema $schema) {
+                        'resolve'     => static function (Schema $schema) : array {
                             return array_values($schema->getTypeMap());
                         },
                     ],
                     'queryType'        => [
                         'description' => 'The type that query operations will be rooted at.',
                         'type'        => new NonNull(self::_type()),
-                        'resolve'     => static function (Schema $schema) {
+                        'resolve'     => static function (Schema $schema) : ?ObjectType {
                             return $schema->getQueryType();
                         },
                     ],
@@ -244,7 +244,7 @@ public static function _schema()
                             'If this server supports mutation, the type that ' .
                             'mutation operations will be rooted at.',
                         'type'        => self::_type(),
-                        'resolve'     => static function (Schema $schema) {
+                        'resolve'     => static function (Schema $schema) : ?ObjectType {
                             return $schema->getMutationType();
                         },
                     ],
@@ -336,7 +336,7 @@ public static function _type()
                                     if (empty($args['includeDeprecated'])) {
                                         $fields = array_filter(
                                             $fields,
-                                            static function (FieldDefinition $field) {
+                                            static function (FieldDefinition $field) : bool {
                                                 return ! $field->deprecationReason;
                                             }
                                         );
@@ -350,7 +350,7 @@ static function (FieldDefinition $field) {
                         ],
                         'interfaces'    => [
                             'type'    => Type::listOf(Type::nonNull(self::_type())),
-                            'resolve' => static function ($type) {
+                            'resolve' => static function ($type) : ?array {
                                 if ($type instanceof ObjectType) {
                                     return $type->getInterfaces();
                                 }
@@ -360,7 +360,7 @@ static function (FieldDefinition $field) {
                         ],
                         'possibleTypes' => [
                             'type'    => Type::listOf(Type::nonNull(self::_type())),
-                            'resolve' => static function ($type, $args, $context, ResolveInfo $info) {
+                            'resolve' => static function ($type, $args, $context, ResolveInfo $info) : ?array {
                                 if ($type instanceof InterfaceType || $type instanceof UnionType) {
                                     return $info->schema->getPossibleTypes($type);
                                 }
@@ -380,7 +380,7 @@ static function (FieldDefinition $field) {
                                     if (empty($args['includeDeprecated'])) {
                                         $values = array_filter(
                                             $values,
-                                            static function ($value) {
+                                            static function ($value) : bool {
                                                 return ! $value->deprecationReason;
                                             }
                                         );
@@ -394,7 +394,7 @@ static function ($value) {
                         ],
                         'inputFields'   => [
                             'type'    => Type::listOf(Type::nonNull(self::_inputValue())),
-                            'resolve' => static function ($type) {
+                            'resolve' => static function ($type) : ?array {
                                 if ($type instanceof InputObjectType) {
                                     return array_values($type->getFields());
                                 }
@@ -404,7 +404,7 @@ static function ($value) {
                         ],
                         'ofType'        => [
                             'type'    => self::_type(),
-                            'resolve' => static function ($type) {
+                            'resolve' => static function ($type) : ?Type {
                                 if ($type instanceof WrappingType) {
                                     return $type->getWrappedType();
                                 }
@@ -480,19 +480,19 @@ public static function _field()
                     return [
                         'name'              => [
                             'type' => Type::nonNull(Type::string()),
-                            'resolve' => static function (FieldDefinition $field) {
+                            'resolve' => static function (FieldDefinition $field) : string {
                                 return $field->name;
                             },
                         ],
                         'description'       => [
                             'type' => Type::string(),
-                            'resolve' => static function (FieldDefinition $field) {
+                            'resolve' => static function (FieldDefinition $field) : ?string {
                                 return $field->description;
                             },
                         ],
                         'args'              => [
                             'type'    => Type::nonNull(Type::listOf(Type::nonNull(self::_inputValue()))),
-                            'resolve' => static function (FieldDefinition $field) {
+                            'resolve' => static function (FieldDefinition $field) : array {
                                 return empty($field->args) ? [] : $field->args;
                             },
                         ],
@@ -504,13 +504,13 @@ public static function _field()
                         ],
                         'isDeprecated'      => [
                             'type'    => Type::nonNull(Type::boolean()),
-                            'resolve' => static function (FieldDefinition $field) {
+                            'resolve' => static function (FieldDefinition $field) : bool {
                                 return (bool) $field->deprecationReason;
                             },
                         ],
                         'deprecationReason' => [
                             'type'    => Type::string(),
-                            'resolve' => static function (FieldDefinition $field) {
+                            'resolve' => static function (FieldDefinition $field) : ?string {
                                 return $field->deprecationReason;
                             },
                         ],
@@ -536,7 +536,7 @@ public static function _inputValue()
                     return [
                         'name'         => [
                             'type' => Type::nonNull(Type::string()),
-                            'resolve' => static function ($inputValue) {
+                            'resolve' => static function ($inputValue) : string {
                                 /** @var FieldArgument|InputObjectField $inputValue */
                                 $inputValue = $inputValue;
 
@@ -545,7 +545,7 @@ public static function _inputValue()
                         ],
                         'description'  => [
                             'type' => Type::string(),
-                            'resolve' => static function ($inputValue) {
+                            'resolve' => static function ($inputValue) : ?string {
                                 /** @var FieldArgument|InputObjectField $inputValue */
                                 $inputValue = $inputValue;
 
@@ -564,7 +564,7 @@ public static function _inputValue()
                             'type'        => Type::string(),
                             'description' =>
                                 'A GraphQL-formatted string representing the default value for this input value.',
-                            'resolve'     => static function ($inputValue) {
+                            'resolve'     => static function ($inputValue) : ?string {
                                 /** @var FieldArgument|InputObjectField $inputValue */
                                 $inputValue = $inputValue;
 
@@ -609,7 +609,7 @@ public static function _enumValue()
                     ],
                     'isDeprecated'      => [
                         'type'    => Type::nonNull(Type::boolean()),
-                        'resolve' => static function ($enumValue) {
+                        'resolve' => static function ($enumValue) : bool {
                             return (bool) $enumValue->deprecationReason;
                         },
                     ],
@@ -661,7 +661,7 @@ public static function _directive()
                     ],
                     'args'        => [
                         'type'    => Type::nonNull(Type::listOf(Type::nonNull(self::_inputValue()))),
-                        'resolve' => static function (Directive $directive) {
+                        'resolve' => static function (Directive $directive) : array {
                             return $directive->args ?: [];
                         },
                     ],
@@ -779,7 +779,7 @@ public static function schemaMetaFieldDef() : FieldDefinition
                     $args,
                     $context,
                     ResolveInfo $info
-                ) {
+                ) : Schema {
                     return $info->schema;
                 },
             ]);
@@ -798,7 +798,7 @@ public static function typeMetaFieldDef() : FieldDefinition
                 'args'        => [
                     ['name' => 'name', 'type' => Type::nonNull(Type::string())],
                 ],
-                'resolve'     => static function ($source, $args, $context, ResolveInfo $info) {
+                'resolve'     => static function ($source, $args, $context, ResolveInfo $info) : Type {
                     return $info->schema->getType($args['name']);
                 },
             ]);
@@ -820,7 +820,7 @@ public static function typeNameMetaFieldDef() : FieldDefinition
                     $args,
                     $context,
                     ResolveInfo $info
-                ) {
+                ) : string {
                     return $info->parentType->name;
                 },
             ]);
diff --git a/src/Type/SchemaValidationContext.php b/src/Type/SchemaValidationContext.php
index dcaee67d0..b37b0990d 100644
--- a/src/Type/SchemaValidationContext.php
+++ b/src/Type/SchemaValidationContext.php
@@ -5,6 +5,7 @@
 namespace GraphQL\Type;
 
 use GraphQL\Error\Error;
+use GraphQL\Language\AST\DirectiveDefinitionNode;
 use GraphQL\Language\AST\DirectiveNode;
 use GraphQL\Language\AST\EnumValueDefinitionNode;
 use GraphQL\Language\AST\FieldDefinitionNode;
@@ -222,7 +223,7 @@ public function validateDirectiveDefinitions()
 
             $nodes = Utils::map(
                 $directiveList,
-                static function (Directive $directive) {
+                static function (Directive $directive) : DirectiveDefinitionNode {
                     return $directive->astNode;
                 }
             );
@@ -263,7 +264,7 @@ static function ($directiveNode) {
 
         return Utils::filter(
             $subNodes,
-            static function ($argNode) use ($argName) {
+            static function ($argNode) use ($argName) : bool {
                 return $argNode->name->value === $argName;
             }
         );
@@ -378,7 +379,7 @@ private function validateDirectivesAtLocation($directives, string $location)
             }
             $includes = Utils::some(
                 $schemaDirective->locations,
-                static function ($schemaLocation) use ($location) {
+                static function ($schemaLocation) use ($location) : bool {
                     return $schemaLocation === $location;
                 }
             );
@@ -566,7 +567,7 @@ private function getAllFieldNodes($type, $fieldName)
             return $typeNode->fields;
         });
 
-        return Utils::filter($subNodes, static function ($fieldNode) use ($fieldName) {
+        return Utils::filter($subNodes, static function ($fieldNode) use ($fieldName) : bool {
             return $fieldNode->name->value === $fieldName;
         });
     }
@@ -711,7 +712,7 @@ private function getAllImplementsInterfaceNodes(ObjectType $type, $iface)
             return $typeNode->interfaces;
         });
 
-        return Utils::filter($subNodes, static function ($ifaceNode) use ($iface) {
+        return Utils::filter($subNodes, static function ($ifaceNode) use ($iface) : bool {
             return $ifaceNode->name->value === $iface->name;
         });
     }
@@ -911,7 +912,7 @@ private function getUnionMemberTypeNodes(UnionType $union, $typeName)
             return $unionNode->types;
         });
 
-        return Utils::filter($subNodes, static function ($typeNode) use ($typeName) {
+        return Utils::filter($subNodes, static function ($typeNode) use ($typeName) : bool {
             return $typeNode->name->value === $typeName;
         });
     }
@@ -971,7 +972,7 @@ private function getEnumValueNodes(EnumType $enum, $valueName)
             return $enumNode->values;
         });
 
-        return Utils::filter($subNodes, static function ($valueNode) use ($valueName) {
+        return Utils::filter($subNodes, static function ($valueNode) use ($valueName) : bool {
             return $valueNode->name->value === $valueName;
         });
     }
diff --git a/src/Utils/ASTDefinitionBuilder.php b/src/Utils/ASTDefinitionBuilder.php
index d9902b1f2..c032442ed 100644
--- a/src/Utils/ASTDefinitionBuilder.php
+++ b/src/Utils/ASTDefinitionBuilder.php
@@ -336,7 +336,7 @@ private function makeImplementedInterfaces(ObjectTypeDefinitionNode $def)
             // validation with validateSchema() will produce more actionable results.
             return Utils::map(
                 $def->interfaces,
-                function ($iface) {
+                function ($iface) : Type {
                     return $this->buildType($iface);
                 }
             );
@@ -370,7 +370,7 @@ private function makeEnumDef(EnumTypeDefinitionNode $def)
                     static function ($enumValue) {
                         return $enumValue->name->value;
                     },
-                    function ($enumValue) {
+                    function ($enumValue) : array {
                         return [
                             'description'       => $this->getDescription($enumValue),
                             'deprecationReason' => $this->getDeprecationReason($enumValue),
@@ -395,7 +395,7 @@ private function makeUnionDef(UnionTypeDefinitionNode $def)
                 ? function () use ($def) {
                     return Utils::map(
                         $def->types,
-                        function ($typeNode) {
+                        function ($typeNode) : Type {
                             return $this->buildType($typeNode);
                         }
                     );
diff --git a/src/Utils/BreakingChangesFinder.php b/src/Utils/BreakingChangesFinder.php
index 733ca1889..8ef9253d2 100644
--- a/src/Utils/BreakingChangesFinder.php
+++ b/src/Utils/BreakingChangesFinder.php
@@ -504,7 +504,7 @@ public static function findArgChanges(
                     $newArgs   = $newTypeFields[$fieldName]->args;
                     $newArgDef = Utils::find(
                         $newArgs,
-                        static function ($arg) use ($oldArgDef) {
+                        static function ($arg) use ($oldArgDef) : bool {
                             return $arg->name === $oldArgDef->name;
                         }
                     );
@@ -544,7 +544,7 @@ static function ($arg) use ($oldArgDef) {
                         $oldArgs   = $oldTypeFields[$fieldName]->args;
                         $oldArgDef = Utils::find(
                             $oldArgs,
-                            static function ($arg) use ($newTypeFieldArgDef) {
+                            static function ($arg) use ($newTypeFieldArgDef) : bool {
                                 return $arg->name === $newTypeFieldArgDef->name;
                             }
                         );
diff --git a/src/Utils/BuildSchema.php b/src/Utils/BuildSchema.php
index 57e9c16f4..fb52b1110 100644
--- a/src/Utils/BuildSchema.php
+++ b/src/Utils/BuildSchema.php
@@ -18,6 +18,7 @@
 use GraphQL\Language\Parser;
 use GraphQL\Language\Source;
 use GraphQL\Type\Definition\Directive;
+use GraphQL\Type\Definition\Type;
 use GraphQL\Type\Schema;
 use GraphQL\Validator\DocumentValidator;
 use function array_map;
@@ -145,7 +146,7 @@ public function buildSchema()
         $DefinitionBuilder = new ASTDefinitionBuilder(
             $this->nodeMap,
             $this->options,
-            static function ($typeName) {
+            static function ($typeName) : void {
                 throw new Error('Type "' . $typeName . '" not found in document.');
             },
             $this->typeConfigDecorator
@@ -189,12 +190,12 @@ static function (Directive $directive) : string {
             'subscription' => isset($operationTypes['subscription'])
                 ? $DefinitionBuilder->buildType($operationTypes['subscription'])
                 : null,
-            'typeLoader'   => static function ($name) use ($DefinitionBuilder) {
+            'typeLoader'   => static function ($name) use ($DefinitionBuilder) : Type {
                 return $DefinitionBuilder->buildType($name);
             },
             'directives'   => $directives,
             'astNode'      => $schemaDef,
-            'types'        => function () use ($DefinitionBuilder) {
+            'types'        => function () use ($DefinitionBuilder) : array {
                 $types = [];
                 /** @var ScalarTypeDefinitionNode|ObjectTypeDefinitionNode|InterfaceTypeDefinitionNode|UnionTypeDefinitionNode|EnumTypeDefinitionNode|InputObjectTypeDefinitionNode $def */
                 foreach ($this->nodeMap as $name => $def) {
diff --git a/src/Utils/SchemaExtender.php b/src/Utils/SchemaExtender.php
index 3249738d2..e24639ba2 100644
--- a/src/Utils/SchemaExtender.php
+++ b/src/Utils/SchemaExtender.php
@@ -141,7 +141,7 @@ protected static function extendUnionType(UnionType $type) : UnionType
         return new UnionType([
             'name' => $type->name,
             'description' => $type->description,
-            'types' => static function () use ($type) {
+            'types' => static function () use ($type) : array {
                 return static::extendPossibleTypes($type);
             },
             'astNode' => $type->astNode,
@@ -166,7 +166,7 @@ protected static function extendInputObjectType(InputObjectType $type) : InputOb
         return new InputObjectType([
             'name' => $type->name,
             'description' => $type->description,
-            'fields' => static function () use ($type) {
+            'fields' => static function () use ($type) : array {
                 return static::extendInputFieldMap($type);
             },
             'astNode' => $type->astNode,
@@ -315,7 +315,7 @@ protected static function extendArgs(array $args) : array
     {
         return Utils::keyValMap(
             $args,
-            static function (FieldArgument $arg) {
+            static function (FieldArgument $arg) : string {
                 return $arg->name;
             },
             static function (FieldArgument $arg) {
@@ -382,10 +382,10 @@ protected static function extendObjectType(ObjectType $type) : ObjectType
         return new ObjectType([
             'name' => $type->name,
             'description' => $type->description,
-            'interfaces' => static function () use ($type) {
+            'interfaces' => static function () use ($type) : array {
                 return static::extendImplementedInterfaces($type);
             },
-            'fields' => static function () use ($type) {
+            'fields' => static function () use ($type) : array {
                 return static::extendFieldMap($type);
             },
             'astNode' => $type->astNode,
@@ -400,7 +400,7 @@ protected static function extendInterfaceType(InterfaceType $type) : InterfaceTy
         return new InterfaceType([
             'name' => $type->name,
             'description' => $type->description,
-            'fields' => static function () use ($type) {
+            'fields' => static function () use ($type) : array {
                 return static::extendFieldMap($type);
             },
             'astNode' => $type->astNode,
@@ -466,7 +466,7 @@ protected static function extendMaybeNamedType(?NamedType $type = null)
      */
     protected static function getMergedDirectives(Schema $schema, array $directiveDefinitions) : array
     {
-        $existingDirectives = array_map(static function (Directive $directive) {
+        $existingDirectives = array_map(static function (Directive $directive) : Directive {
             return static::extendDirective($directive);
         }, $schema->getDirectives());
 
@@ -619,7 +619,7 @@ static function (string $typeName) use ($schema) {
                 return static::extendNamedType($type);
             }, array_values($schema->getTypeMap())),
             // Do the same with new types.
-            array_map(static function ($type) {
+            array_map(static function ($type) : Type {
                 return static::$astBuilder->buildType($type);
             }, array_values($typeDefinitionMap))
         );
diff --git a/src/Utils/SchemaPrinter.php b/src/Utils/SchemaPrinter.php
index ccbb00734..795c4cc2d 100644
--- a/src/Utils/SchemaPrinter.php
+++ b/src/Utils/SchemaPrinter.php
@@ -51,10 +51,10 @@ public static function doPrint(Schema $schema, array $options = []) : string
     {
         return self::printFilteredSchema(
             $schema,
-            static function ($type) {
+            static function ($type) : bool {
                 return ! Directive::isSpecifiedDirective($type);
             },
-            static function ($type) {
+            static function ($type) : bool {
                 return ! Type::isBuiltInType($type);
             },
             $options
@@ -85,13 +85,13 @@ static function ($directive) use ($directiveFilter) {
                     array_merge(
                         [self::printSchemaDefinition($schema)],
                         array_map(
-                            static function ($directive) use ($options) {
+                            static function ($directive) use ($options) : string {
                                 return self::printDirective($directive, $options);
                             },
                             $directives
                         ),
                         array_map(
-                            static function ($type) use ($options) {
+                            static function ($type) use ($options) : string {
                                 return self::printType($type, $options);
                             },
                             $types
@@ -273,7 +273,7 @@ private static function printArgs($options, $args, $indentation = '') : string
         // If every arg does not have a description, print them on one line.
         if (Utils::every(
             $args,
-            static function ($arg) {
+            static function ($arg) : bool {
                 return empty($arg->description);
             }
         )) {
@@ -285,7 +285,7 @@ static function ($arg) {
             implode(
                 "\n",
                 array_map(
-                    static function ($arg, $i) use ($indentation, $options) {
+                    static function ($arg, $i) use ($indentation, $options) : string {
                         return self::printDescription($options, $arg, '  ' . $indentation, ! $i) . '  ' . $indentation .
                             self::printInputValue($arg);
                     },
@@ -357,7 +357,7 @@ private static function printObject(ObjectType $type, array $options) : string
             ? ' implements ' . implode(
                 ' & ',
                 array_map(
-                    static function ($i) {
+                    static function ($i) : string {
                         return $i->name;
                     },
                     $interfaces
@@ -379,7 +379,7 @@ private static function printFields($options, $type) : string
         return implode(
             "\n",
             array_map(
-                static function ($f, $i) use ($options) {
+                static function ($f, $i) use ($options) : string {
                     return self::printDescription($options, $f, '  ', ! $i) . '  ' .
                         $f->name . self::printArgs($options, $f->args, '  ') . ': ' .
                         (string) $f->getType() . self::printDeprecated($f);
@@ -439,7 +439,7 @@ private static function printEnumValues($values, $options) : string
         return implode(
             "\n",
             array_map(
-                static function ($value, $i) use ($options) {
+                static function ($value, $i) use ($options) : string {
                     return self::printDescription($options, $value, '  ', ! $i) . '  ' .
                         $value->name . self::printDeprecated($value);
                 },
@@ -463,7 +463,7 @@ private static function printInputObject(InputObjectType $type, array $options)
                 implode(
                     "\n",
                     array_map(
-                        static function ($f, $i) use ($options) {
+                        static function ($f, $i) use ($options) : string {
                             return self::printDescription($options, $f, '  ', ! $i) . '  ' . self::printInputValue($f);
                         },
                         $fields,
diff --git a/src/Utils/TypeInfo.php b/src/Utils/TypeInfo.php
index 1468847f9..870180115 100644
--- a/src/Utils/TypeInfo.php
+++ b/src/Utils/TypeInfo.php
@@ -323,7 +323,7 @@ public function enter(Node $node)
                     /** @var FieldArgument $argDef */
                     $argDef = Utils::find(
                         $fieldOrDirective->args,
-                        static function ($arg) use ($node) {
+                        static function ($arg) use ($node) : bool {
                             return $arg->name === $node->name->value;
                         }
                     );
diff --git a/src/Utils/Utils.php b/src/Utils/Utils.php
index 9f0defa55..7919f262e 100644
--- a/src/Utils/Utils.php
+++ b/src/Utils/Utils.php
@@ -560,7 +560,7 @@ public static function withErrorHandling(callable $fn, array &$errors)
     {
         return static function () use ($fn, &$errors) {
             // Catch custom errors (to report them in query results)
-            set_error_handler(static function ($severity, $message, $file, $line) use (&$errors) {
+            set_error_handler(static function ($severity, $message, $file, $line) use (&$errors) : void {
                 $errors[] = new ErrorException($message, 0, $severity, $file, $line);
             });
 
@@ -580,7 +580,7 @@ public static function withErrorHandling(callable $fn, array &$errors)
     public static function quotedOrList(array $items)
     {
         $items = array_map(
-            static function ($item) {
+            static function ($item) : string {
                 return sprintf('"%s"', $item);
             },
             $items
@@ -609,7 +609,7 @@ public static function orList(array $items)
 
         return array_reduce(
             range(1, $selectedLength - 1),
-            static function ($list, $index) use ($selected, $selectedLength) {
+            static function ($list, $index) use ($selected, $selectedLength) : string {
                 return $list .
                     ($selectedLength > 2 ? ', ' : ' ') .
                     ($index === $selectedLength - 1 ? 'or ' : '') .
diff --git a/src/Utils/Value.php b/src/Utils/Value.php
index 595172a25..2920883a6 100644
--- a/src/Utils/Value.php
+++ b/src/Utils/Value.php
@@ -90,7 +90,7 @@ public static function coerceValue($value, InputType $type, $blameNode = null, ?
             $suggestions = Utils::suggestionList(
                 Utils::printSafe($value),
                 array_map(
-                    static function ($enumValue) {
+                    static function ($enumValue) : string {
                         return $enumValue->name;
                     },
                     $type->getValues()
diff --git a/src/Validator/DocumentValidator.php b/src/Validator/DocumentValidator.php
index 7df559621..0b5fb0c1b 100644
--- a/src/Validator/DocumentValidator.php
+++ b/src/Validator/DocumentValidator.php
@@ -264,7 +264,7 @@ public static function isError($value)
         return is_array($value)
             ? count(array_filter(
                 $value,
-                static function ($item) {
+                static function ($item) : bool {
                     return $item instanceof Throwable;
                 }
             )) === count($value)
diff --git a/src/Validator/Rules/ExecutableDefinitions.php b/src/Validator/Rules/ExecutableDefinitions.php
index 632953e8a..3d1887daf 100644
--- a/src/Validator/Rules/ExecutableDefinitions.php
+++ b/src/Validator/Rules/ExecutableDefinitions.php
@@ -11,6 +11,7 @@
 use GraphQL\Language\AST\OperationDefinitionNode;
 use GraphQL\Language\AST\TypeSystemDefinitionNode;
 use GraphQL\Language\Visitor;
+use GraphQL\Language\VisitorOperation;
 use GraphQL\Validator\ValidationContext;
 use function sprintf;
 
@@ -25,7 +26,7 @@ class ExecutableDefinitions extends ValidationRule
     public function getVisitor(ValidationContext $context)
     {
         return [
-            NodeKind::DOCUMENT => static function (DocumentNode $node) use ($context) {
+            NodeKind::DOCUMENT => static function (DocumentNode $node) use ($context) : VisitorOperation {
                 /** @var FragmentDefinitionNode|OperationDefinitionNode|TypeSystemDefinitionNode $definition */
                 foreach ($node->definitions as $definition) {
                     if ($definition instanceof OperationDefinitionNode ||
diff --git a/src/Validator/Rules/FieldsOnCorrectType.php b/src/Validator/Rules/FieldsOnCorrectType.php
index 105325536..55b66eeb6 100644
--- a/src/Validator/Rules/FieldsOnCorrectType.php
+++ b/src/Validator/Rules/FieldsOnCorrectType.php
@@ -23,7 +23,7 @@ class FieldsOnCorrectType extends ValidationRule
     public function getVisitor(ValidationContext $context)
     {
         return [
-            NodeKind::FIELD => function (FieldNode $node) use ($context) {
+            NodeKind::FIELD => function (FieldNode $node) use ($context) : void {
                 $type = $context->getParentType();
                 if (! $type) {
                     return;
diff --git a/src/Validator/Rules/FragmentsOnCompositeTypes.php b/src/Validator/Rules/FragmentsOnCompositeTypes.php
index c5ca80771..db72a057b 100644
--- a/src/Validator/Rules/FragmentsOnCompositeTypes.php
+++ b/src/Validator/Rules/FragmentsOnCompositeTypes.php
@@ -19,7 +19,7 @@ class FragmentsOnCompositeTypes extends ValidationRule
     public function getVisitor(ValidationContext $context)
     {
         return [
-            NodeKind::INLINE_FRAGMENT     => static function (InlineFragmentNode $node) use ($context) {
+            NodeKind::INLINE_FRAGMENT     => static function (InlineFragmentNode $node) use ($context) : void {
                 if (! $node->typeCondition) {
                     return;
                 }
@@ -34,7 +34,7 @@ public function getVisitor(ValidationContext $context)
                     [$node->typeCondition]
                 ));
             },
-            NodeKind::FRAGMENT_DEFINITION => static function (FragmentDefinitionNode $node) use ($context) {
+            NodeKind::FRAGMENT_DEFINITION => static function (FragmentDefinitionNode $node) use ($context) : void {
                 $type = TypeInfo::typeFromAST($context->getSchema(), $node->typeCondition);
 
                 if (! $type || Type::isCompositeType($type)) {
diff --git a/src/Validator/Rules/KnownArgumentNames.php b/src/Validator/Rules/KnownArgumentNames.php
index 0a4945ebd..685f4176f 100644
--- a/src/Validator/Rules/KnownArgumentNames.php
+++ b/src/Validator/Rules/KnownArgumentNames.php
@@ -48,7 +48,7 @@ public function getVisitor(ValidationContext $context)
                                 Utils::suggestionList(
                                     $node->name->value,
                                     array_map(
-                                        static function ($arg) {
+                                        static function ($arg) : string {
                                             return $arg->name;
                                         },
                                         $fieldDef->args
diff --git a/src/Validator/Rules/KnownArgumentNamesOnDirectives.php b/src/Validator/Rules/KnownArgumentNamesOnDirectives.php
index 4087a321a..e636482c6 100644
--- a/src/Validator/Rules/KnownArgumentNamesOnDirectives.php
+++ b/src/Validator/Rules/KnownArgumentNamesOnDirectives.php
@@ -68,7 +68,7 @@ static function (FieldArgument $arg) : string {
         }
 
         return [
-            NodeKind::DIRECTIVE => static function (DirectiveNode $directiveNode) use ($directiveArgs, $context) {
+            NodeKind::DIRECTIVE => static function (DirectiveNode $directiveNode) use ($directiveArgs, $context) : void {
                 $directiveName = $directiveNode->name->value;
                 $knownArgs     = $directiveArgs[$directiveName] ?? null;
 
diff --git a/src/Validator/Rules/KnownFragmentNames.php b/src/Validator/Rules/KnownFragmentNames.php
index e26e233e0..052686f28 100644
--- a/src/Validator/Rules/KnownFragmentNames.php
+++ b/src/Validator/Rules/KnownFragmentNames.php
@@ -15,7 +15,7 @@ class KnownFragmentNames extends ValidationRule
     public function getVisitor(ValidationContext $context)
     {
         return [
-            NodeKind::FRAGMENT_SPREAD => static function (FragmentSpreadNode $node) use ($context) {
+            NodeKind::FRAGMENT_SPREAD => static function (FragmentSpreadNode $node) use ($context) : void {
                 $fragmentName = $node->name->value;
                 $fragment     = $context->getFragment($fragmentName);
                 if ($fragment) {
diff --git a/src/Validator/Rules/KnownTypeNames.php b/src/Validator/Rules/KnownTypeNames.php
index 9abaf0aeb..6f38e237d 100644
--- a/src/Validator/Rules/KnownTypeNames.php
+++ b/src/Validator/Rules/KnownTypeNames.php
@@ -8,6 +8,7 @@
 use GraphQL\Language\AST\NamedTypeNode;
 use GraphQL\Language\AST\NodeKind;
 use GraphQL\Language\Visitor;
+use GraphQL\Language\VisitorOperation;
 use GraphQL\Utils\Utils;
 use GraphQL\Validator\ValidationContext;
 use function array_keys;
@@ -23,7 +24,7 @@ class KnownTypeNames extends ValidationRule
 {
     public function getVisitor(ValidationContext $context)
     {
-        $skip = static function () {
+        $skip = static function () : VisitorOperation {
             return Visitor::skipNode();
         };
 
@@ -35,7 +36,7 @@ public function getVisitor(ValidationContext $context)
             NodeKind::INTERFACE_TYPE_DEFINITION    => $skip,
             NodeKind::UNION_TYPE_DEFINITION        => $skip,
             NodeKind::INPUT_OBJECT_TYPE_DEFINITION => $skip,
-            NodeKind::NAMED_TYPE                   => static function (NamedTypeNode $node) use ($context) {
+            NodeKind::NAMED_TYPE                   => static function (NamedTypeNode $node) use ($context) : void {
                 $schema   = $context->getSchema();
                 $typeName = $node->name->value;
                 $type     = $schema->getType($typeName);
diff --git a/src/Validator/Rules/LoneAnonymousOperation.php b/src/Validator/Rules/LoneAnonymousOperation.php
index ebfba6299..f0a68cf6e 100644
--- a/src/Validator/Rules/LoneAnonymousOperation.php
+++ b/src/Validator/Rules/LoneAnonymousOperation.php
@@ -26,10 +26,10 @@ public function getVisitor(ValidationContext $context)
         $operationCount = 0;
 
         return [
-            NodeKind::DOCUMENT             => static function (DocumentNode $node) use (&$operationCount) {
+            NodeKind::DOCUMENT             => static function (DocumentNode $node) use (&$operationCount) : void {
                 $tmp = Utils::filter(
                     $node->definitions,
-                    static function (Node $definition) {
+                    static function (Node $definition) : bool {
                         return $definition instanceof OperationDefinitionNode;
                     }
                 );
diff --git a/src/Validator/Rules/LoneSchemaDefinition.php b/src/Validator/Rules/LoneSchemaDefinition.php
index bd89f8c46..4ece976b6 100644
--- a/src/Validator/Rules/LoneSchemaDefinition.php
+++ b/src/Validator/Rules/LoneSchemaDefinition.php
@@ -41,7 +41,7 @@ public function getSDLVisitor(SDLValidationContext $context)
         $schemaDefinitionsCount = 0;
 
         return [
-            NodeKind::SCHEMA_DEFINITION => static function (SchemaDefinitionNode $node) use ($alreadyDefined, $context, &$schemaDefinitionsCount) {
+            NodeKind::SCHEMA_DEFINITION => static function (SchemaDefinitionNode $node) use ($alreadyDefined, $context, &$schemaDefinitionsCount) : void {
                 if ($alreadyDefined !== false) {
                     $context->reportError(new Error(self::canNotDefineSchemaWithinExtensionMessage(), $node));
 
diff --git a/src/Validator/Rules/NoFragmentCycles.php b/src/Validator/Rules/NoFragmentCycles.php
index e01b7631d..a7f938018 100644
--- a/src/Validator/Rules/NoFragmentCycles.php
+++ b/src/Validator/Rules/NoFragmentCycles.php
@@ -9,6 +9,7 @@
 use GraphQL\Language\AST\FragmentSpreadNode;
 use GraphQL\Language\AST\NodeKind;
 use GraphQL\Language\Visitor;
+use GraphQL\Language\VisitorOperation;
 use GraphQL\Utils\Utils;
 use GraphQL\Validator\ValidationContext;
 use function array_pop;
@@ -41,10 +42,10 @@ public function getVisitor(ValidationContext $context)
         $this->spreadPathIndexByName = [];
 
         return [
-            NodeKind::OPERATION_DEFINITION => static function () {
+            NodeKind::OPERATION_DEFINITION => static function () : VisitorOperation {
                 return Visitor::skipNode();
             },
-            NodeKind::FRAGMENT_DEFINITION  => function (FragmentDefinitionNode $node) use ($context) {
+            NodeKind::FRAGMENT_DEFINITION  => function (FragmentDefinitionNode $node) use ($context) : VisitorOperation {
                 $this->detectCycleRecursive($node, $context);
 
                 return Visitor::skipNode();
diff --git a/src/Validator/Rules/NoUndefinedVariables.php b/src/Validator/Rules/NoUndefinedVariables.php
index f59d03f6a..5dbba94a6 100644
--- a/src/Validator/Rules/NoUndefinedVariables.php
+++ b/src/Validator/Rules/NoUndefinedVariables.php
@@ -23,10 +23,10 @@ public function getVisitor(ValidationContext $context)
 
         return [
             NodeKind::OPERATION_DEFINITION => [
-                'enter' => static function () use (&$variableNameDefined) {
+                'enter' => static function () use (&$variableNameDefined) : void {
                     $variableNameDefined = [];
                 },
-                'leave' => static function (OperationDefinitionNode $operation) use (&$variableNameDefined, $context) {
+                'leave' => static function (OperationDefinitionNode $operation) use (&$variableNameDefined, $context) : void {
                     $usages = $context->getRecursiveVariableUsages($operation);
 
                     foreach ($usages as $usage) {
@@ -49,7 +49,7 @@ public function getVisitor(ValidationContext $context)
                     }
                 },
             ],
-            NodeKind::VARIABLE_DEFINITION  => static function (VariableDefinitionNode $def) use (&$variableNameDefined) {
+            NodeKind::VARIABLE_DEFINITION  => static function (VariableDefinitionNode $def) use (&$variableNameDefined) : void {
                 $variableNameDefined[$def->variable->name->value] = true;
             },
         ];
diff --git a/src/Validator/Rules/NoUnusedFragments.php b/src/Validator/Rules/NoUnusedFragments.php
index d1cd3668f..6c3f19823 100644
--- a/src/Validator/Rules/NoUnusedFragments.php
+++ b/src/Validator/Rules/NoUnusedFragments.php
@@ -9,6 +9,7 @@
 use GraphQL\Language\AST\NodeKind;
 use GraphQL\Language\AST\OperationDefinitionNode;
 use GraphQL\Language\Visitor;
+use GraphQL\Language\VisitorOperation;
 use GraphQL\Validator\ValidationContext;
 use function sprintf;
 
@@ -26,18 +27,18 @@ public function getVisitor(ValidationContext $context)
         $this->fragmentDefs  = [];
 
         return [
-            NodeKind::OPERATION_DEFINITION => function ($node) {
+            NodeKind::OPERATION_DEFINITION => function ($node) : VisitorOperation {
                 $this->operationDefs[] = $node;
 
                 return Visitor::skipNode();
             },
-            NodeKind::FRAGMENT_DEFINITION  => function (FragmentDefinitionNode $def) {
+            NodeKind::FRAGMENT_DEFINITION  => function (FragmentDefinitionNode $def) : VisitorOperation {
                 $this->fragmentDefs[] = $def;
 
                 return Visitor::skipNode();
             },
             NodeKind::DOCUMENT             => [
-                'leave' => function () use ($context) {
+                'leave' => function () use ($context) : void {
                     $fragmentNameUsed = [];
 
                     foreach ($this->operationDefs as $operation) {
diff --git a/src/Validator/Rules/NoUnusedVariables.php b/src/Validator/Rules/NoUnusedVariables.php
index d6a4486d7..592279cd3 100644
--- a/src/Validator/Rules/NoUnusedVariables.php
+++ b/src/Validator/Rules/NoUnusedVariables.php
@@ -22,10 +22,10 @@ public function getVisitor(ValidationContext $context)
 
         return [
             NodeKind::OPERATION_DEFINITION => [
-                'enter' => function () {
+                'enter' => function () : void {
                     $this->variableDefs = [];
                 },
-                'leave' => function (OperationDefinitionNode $operation) use ($context) {
+                'leave' => function (OperationDefinitionNode $operation) use ($context) : void {
                     $variableNameUsed = [];
                     $usages           = $context->getRecursiveVariableUsages($operation);
                     $opName           = $operation->name !== null
@@ -51,7 +51,7 @@ public function getVisitor(ValidationContext $context)
                     }
                 },
             ],
-            NodeKind::VARIABLE_DEFINITION  => function ($def) {
+            NodeKind::VARIABLE_DEFINITION  => function ($def) : void {
                 $this->variableDefs[] = $def;
             },
         ];
diff --git a/src/Validator/Rules/OverlappingFieldsCanBeMerged.php b/src/Validator/Rules/OverlappingFieldsCanBeMerged.php
index 350318867..790c28cf9 100644
--- a/src/Validator/Rules/OverlappingFieldsCanBeMerged.php
+++ b/src/Validator/Rules/OverlappingFieldsCanBeMerged.php
@@ -59,7 +59,7 @@ public function getVisitor(ValidationContext $context)
         $this->cachedFieldsAndFragmentNames = new SplObjectStorage();
 
         return [
-            NodeKind::SELECTION_SET => function (SelectionSetNode $selectionSet) use ($context) {
+            NodeKind::SELECTION_SET => function (SelectionSetNode $selectionSet) use ($context) : void {
                 $conflicts = $this->findConflictsWithinSelectionSet(
                     $context,
                     $context->getParentType(),
@@ -845,14 +845,14 @@ static function ($conflict) {
             ],
             array_reduce(
                 $conflicts,
-                static function ($allFields, $conflict) {
+                static function ($allFields, $conflict) : array {
                     return array_merge($allFields, $conflict[1]);
                 },
                 [$ast1]
             ),
             array_reduce(
                 $conflicts,
-                static function ($allFields, $conflict) {
+                static function ($allFields, $conflict) : array {
                     return array_merge($allFields, $conflict[2]);
                 },
                 [$ast2]
@@ -879,7 +879,7 @@ public static function reasonMessage($reason)
     {
         if (is_array($reason)) {
             $tmp = array_map(
-                static function ($tmp) {
+                static function ($tmp) : string {
                     [$responseName, $subReason] = $tmp;
 
                     $reasonMessage = self::reasonMessage($subReason);
diff --git a/src/Validator/Rules/PossibleFragmentSpreads.php b/src/Validator/Rules/PossibleFragmentSpreads.php
index 26611e6da..184f58bec 100644
--- a/src/Validator/Rules/PossibleFragmentSpreads.php
+++ b/src/Validator/Rules/PossibleFragmentSpreads.php
@@ -23,7 +23,7 @@ class PossibleFragmentSpreads extends ValidationRule
     public function getVisitor(ValidationContext $context)
     {
         return [
-            NodeKind::INLINE_FRAGMENT => function (InlineFragmentNode $node) use ($context) {
+            NodeKind::INLINE_FRAGMENT => function (InlineFragmentNode $node) use ($context) : void {
                 $fragType   = $context->getType();
                 $parentType = $context->getParentType();
 
@@ -38,7 +38,7 @@ public function getVisitor(ValidationContext $context)
                     [$node]
                 ));
             },
-            NodeKind::FRAGMENT_SPREAD => function (FragmentSpreadNode $node) use ($context) {
+            NodeKind::FRAGMENT_SPREAD => function (FragmentSpreadNode $node) use ($context) : void {
                 $fragName   = $node->name->value;
                 $fragType   = $this->getFragmentType($context, $fragName);
                 $parentType = $context->getParentType();
diff --git a/src/Validator/Rules/ProvidedRequiredArguments.php b/src/Validator/Rules/ProvidedRequiredArguments.php
index 2099f3c48..41730ebe2 100644
--- a/src/Validator/Rules/ProvidedRequiredArguments.php
+++ b/src/Validator/Rules/ProvidedRequiredArguments.php
@@ -9,6 +9,7 @@
 use GraphQL\Language\AST\FieldNode;
 use GraphQL\Language\AST\NodeKind;
 use GraphQL\Language\Visitor;
+use GraphQL\Language\VisitorOperation;
 use GraphQL\Type\Definition\NonNull;
 use GraphQL\Validator\ValidationContext;
 use function sprintf;
@@ -19,7 +20,7 @@ public function getVisitor(ValidationContext $context)
     {
         return [
             NodeKind::FIELD     => [
-                'leave' => static function (FieldNode $fieldNode) use ($context) {
+                'leave' => static function (FieldNode $fieldNode) use ($context) : ?VisitorOperation {
                     $fieldDef = $context->getFieldDef();
 
                     if (! $fieldDef) {
@@ -42,10 +43,12 @@ public function getVisitor(ValidationContext $context)
                             [$fieldNode]
                         ));
                     }
+
+                    return null;
                 },
             ],
             NodeKind::DIRECTIVE => [
-                'leave' => static function (DirectiveNode $directiveNode) use ($context) {
+                'leave' => static function (DirectiveNode $directiveNode) use ($context) : ?VisitorOperation {
                     $directiveDef = $context->getDirective();
                     if (! $directiveDef) {
                         return Visitor::skipNode();
@@ -71,6 +74,8 @@ public function getVisitor(ValidationContext $context)
                             [$directiveNode]
                         ));
                     }
+
+                    return null;
                 },
             ],
         ];
diff --git a/src/Validator/Rules/ProvidedRequiredArgumentsOnDirectives.php b/src/Validator/Rules/ProvidedRequiredArgumentsOnDirectives.php
index c26c061c6..2eacdc202 100644
--- a/src/Validator/Rules/ProvidedRequiredArgumentsOnDirectives.php
+++ b/src/Validator/Rules/ProvidedRequiredArgumentsOnDirectives.php
@@ -85,7 +85,7 @@ static function (NamedTypeNode $argument) : string {
         }
 
         return [
-            NodeKind::DIRECTIVE => static function (DirectiveNode $directiveNode) use ($requiredArgsMap, $context) {
+            NodeKind::DIRECTIVE => static function (DirectiveNode $directiveNode) use ($requiredArgsMap, $context) : ?string {
                 $directiveName = $directiveNode->name->value;
                 $requiredArgs  = $requiredArgsMap[$directiveName] ?? null;
                 if (! $requiredArgs) {
@@ -109,6 +109,8 @@ static function (ArgumentNode $arg) : string {
                         new Error(static::missingDirectiveArgMessage($directiveName, $argName), [$directiveNode])
                     );
                 }
+
+                return null;
             },
         ];
     }
diff --git a/src/Validator/Rules/QueryComplexity.php b/src/Validator/Rules/QueryComplexity.php
index 7d98c2e25..b084799b3 100644
--- a/src/Validator/Rules/QueryComplexity.php
+++ b/src/Validator/Rules/QueryComplexity.php
@@ -15,6 +15,7 @@
 use GraphQL\Language\AST\OperationDefinitionNode;
 use GraphQL\Language\AST\SelectionSetNode;
 use GraphQL\Language\Visitor;
+use GraphQL\Language\VisitorOperation;
 use GraphQL\Type\Definition\Directive;
 use GraphQL\Type\Definition\FieldDefinition;
 use GraphQL\Validator\ValidationContext;
@@ -60,7 +61,7 @@ public function getVisitor(ValidationContext $context)
         return $this->invokeIfNeeded(
             $context,
             [
-                NodeKind::SELECTION_SET        => function (SelectionSetNode $selectionSet) use ($context) {
+                NodeKind::SELECTION_SET        => function (SelectionSetNode $selectionSet) use ($context) : void {
                     $this->fieldNodeAndDefs = $this->collectFieldASTsAndDefs(
                         $context,
                         $context->getParentType(),
@@ -69,13 +70,13 @@ public function getVisitor(ValidationContext $context)
                         $this->fieldNodeAndDefs
                     );
                 },
-                NodeKind::VARIABLE_DEFINITION  => function ($def) {
+                NodeKind::VARIABLE_DEFINITION  => function ($def) : VisitorOperation {
                     $this->variableDefs[] = $def;
 
                     return Visitor::skipNode();
                 },
                 NodeKind::OPERATION_DEFINITION => [
-                    'leave' => function (OperationDefinitionNode $operationDefinition) use ($context, &$complexity) {
+                    'leave' => function (OperationDefinitionNode $operationDefinition) use ($context, &$complexity) : void {
                         $errors = $context->getErrors();
 
                         if (! empty($errors)) {
diff --git a/src/Validator/Rules/QueryDepth.php b/src/Validator/Rules/QueryDepth.php
index cb01dd235..9b83d061e 100644
--- a/src/Validator/Rules/QueryDepth.php
+++ b/src/Validator/Rules/QueryDepth.php
@@ -31,7 +31,7 @@ public function getVisitor(ValidationContext $context)
             $context,
             [
                 NodeKind::OPERATION_DEFINITION => [
-                    'leave' => function (OperationDefinitionNode $operationDefinition) use ($context) {
+                    'leave' => function (OperationDefinitionNode $operationDefinition) use ($context) : void {
                         $maxDepth = $this->fieldDepth($operationDefinition);
 
                         if ($maxDepth <= $this->getMaxQueryDepth()) {
diff --git a/src/Validator/Rules/ScalarLeafs.php b/src/Validator/Rules/ScalarLeafs.php
index 1bd25152b..f1714239e 100644
--- a/src/Validator/Rules/ScalarLeafs.php
+++ b/src/Validator/Rules/ScalarLeafs.php
@@ -16,7 +16,7 @@ class ScalarLeafs extends ValidationRule
     public function getVisitor(ValidationContext $context)
     {
         return [
-            NodeKind::FIELD => static function (FieldNode $node) use ($context) {
+            NodeKind::FIELD => static function (FieldNode $node) use ($context) : void {
                 $type = $context->getType();
                 if (! $type) {
                     return;
diff --git a/src/Validator/Rules/UniqueArgumentNames.php b/src/Validator/Rules/UniqueArgumentNames.php
index 982fd6346..f3010bc77 100644
--- a/src/Validator/Rules/UniqueArgumentNames.php
+++ b/src/Validator/Rules/UniqueArgumentNames.php
@@ -9,6 +9,7 @@
 use GraphQL\Language\AST\NameNode;
 use GraphQL\Language\AST\NodeKind;
 use GraphQL\Language\Visitor;
+use GraphQL\Language\VisitorOperation;
 use GraphQL\Validator\ASTValidationContext;
 use GraphQL\Validator\SDLValidationContext;
 use GraphQL\Validator\ValidationContext;
@@ -34,13 +35,13 @@ public function getASTVisitor(ASTValidationContext $context)
         $this->knownArgNames = [];
 
         return [
-            NodeKind::FIELD     => function () {
+            NodeKind::FIELD     => function () : void {
                 $this->knownArgNames = [];
             },
-            NodeKind::DIRECTIVE => function () {
+            NodeKind::DIRECTIVE => function () : void {
                 $this->knownArgNames = [];
             },
-            NodeKind::ARGUMENT  => function (ArgumentNode $node) use ($context) {
+            NodeKind::ARGUMENT  => function (ArgumentNode $node) use ($context) : VisitorOperation {
                 $argName = $node->name->value;
                 if (! empty($this->knownArgNames[$argName])) {
                     $context->reportError(new Error(
diff --git a/src/Validator/Rules/UniqueDirectivesPerLocation.php b/src/Validator/Rules/UniqueDirectivesPerLocation.php
index 9b784f19c..6860222d5 100644
--- a/src/Validator/Rules/UniqueDirectivesPerLocation.php
+++ b/src/Validator/Rules/UniqueDirectivesPerLocation.php
@@ -27,7 +27,7 @@ public function getSDLVisitor(SDLValidationContext $context)
     public function getASTVisitor(ASTValidationContext $context)
     {
         return [
-            'enter' => static function (Node $node) use ($context) {
+            'enter' => static function (Node $node) use ($context) : void {
                 if (! isset($node->directives)) {
                     return;
                 }
diff --git a/src/Validator/Rules/UniqueFragmentNames.php b/src/Validator/Rules/UniqueFragmentNames.php
index 475e6b3fa..6333a10c2 100644
--- a/src/Validator/Rules/UniqueFragmentNames.php
+++ b/src/Validator/Rules/UniqueFragmentNames.php
@@ -9,6 +9,7 @@
 use GraphQL\Language\AST\NameNode;
 use GraphQL\Language\AST\NodeKind;
 use GraphQL\Language\Visitor;
+use GraphQL\Language\VisitorOperation;
 use GraphQL\Validator\ValidationContext;
 use function sprintf;
 
@@ -22,10 +23,10 @@ public function getVisitor(ValidationContext $context)
         $this->knownFragmentNames = [];
 
         return [
-            NodeKind::OPERATION_DEFINITION => static function () {
+            NodeKind::OPERATION_DEFINITION => static function () : VisitorOperation {
                 return Visitor::skipNode();
             },
-            NodeKind::FRAGMENT_DEFINITION  => function (FragmentDefinitionNode $node) use ($context) {
+            NodeKind::FRAGMENT_DEFINITION  => function (FragmentDefinitionNode $node) use ($context) : VisitorOperation {
                 $fragmentName = $node->name->value;
                 if (empty($this->knownFragmentNames[$fragmentName])) {
                     $this->knownFragmentNames[$fragmentName] = $node->name;
diff --git a/src/Validator/Rules/UniqueInputFieldNames.php b/src/Validator/Rules/UniqueInputFieldNames.php
index da36b9bf8..5eab0260d 100644
--- a/src/Validator/Rules/UniqueInputFieldNames.php
+++ b/src/Validator/Rules/UniqueInputFieldNames.php
@@ -9,6 +9,7 @@
 use GraphQL\Language\AST\NodeKind;
 use GraphQL\Language\AST\ObjectFieldNode;
 use GraphQL\Language\Visitor;
+use GraphQL\Language\VisitorOperation;
 use GraphQL\Validator\ASTValidationContext;
 use GraphQL\Validator\SDLValidationContext;
 use GraphQL\Validator\ValidationContext;
@@ -40,15 +41,15 @@ public function getASTVisitor(ASTValidationContext $context)
 
         return [
             NodeKind::OBJECT       => [
-                'enter' => function () {
+                'enter' => function () : void {
                     $this->knownNameStack[] = $this->knownNames;
                     $this->knownNames       = [];
                 },
-                'leave' => function () {
+                'leave' => function () : void {
                     $this->knownNames = array_pop($this->knownNameStack);
                 },
             ],
-            NodeKind::OBJECT_FIELD => function (ObjectFieldNode $node) use ($context) {
+            NodeKind::OBJECT_FIELD => function (ObjectFieldNode $node) use ($context) : VisitorOperation {
                 $fieldName = $node->name->value;
 
                 if (! empty($this->knownNames[$fieldName])) {
diff --git a/src/Validator/Rules/UniqueOperationNames.php b/src/Validator/Rules/UniqueOperationNames.php
index 761ce2cba..bc9c19910 100644
--- a/src/Validator/Rules/UniqueOperationNames.php
+++ b/src/Validator/Rules/UniqueOperationNames.php
@@ -9,6 +9,7 @@
 use GraphQL\Language\AST\NodeKind;
 use GraphQL\Language\AST\OperationDefinitionNode;
 use GraphQL\Language\Visitor;
+use GraphQL\Language\VisitorOperation;
 use GraphQL\Validator\ValidationContext;
 use function sprintf;
 
@@ -22,7 +23,7 @@ public function getVisitor(ValidationContext $context)
         $this->knownOperationNames = [];
 
         return [
-            NodeKind::OPERATION_DEFINITION => function (OperationDefinitionNode $node) use ($context) {
+            NodeKind::OPERATION_DEFINITION => function (OperationDefinitionNode $node) use ($context) : VisitorOperation {
                 $operationName = $node->name;
 
                 if ($operationName !== null) {
@@ -38,7 +39,7 @@ public function getVisitor(ValidationContext $context)
 
                 return Visitor::skipNode();
             },
-            NodeKind::FRAGMENT_DEFINITION  => static function () {
+            NodeKind::FRAGMENT_DEFINITION  => static function () : VisitorOperation {
                 return Visitor::skipNode();
             },
         ];
diff --git a/src/Validator/Rules/UniqueVariableNames.php b/src/Validator/Rules/UniqueVariableNames.php
index f050a7342..8292e845a 100644
--- a/src/Validator/Rules/UniqueVariableNames.php
+++ b/src/Validator/Rules/UniqueVariableNames.php
@@ -21,10 +21,10 @@ public function getVisitor(ValidationContext $context)
         $this->knownVariableNames = [];
 
         return [
-            NodeKind::OPERATION_DEFINITION => function () {
+            NodeKind::OPERATION_DEFINITION => function () : void {
                 $this->knownVariableNames = [];
             },
-            NodeKind::VARIABLE_DEFINITION  => function (VariableDefinitionNode $node) use ($context) {
+            NodeKind::VARIABLE_DEFINITION  => function (VariableDefinitionNode $node) use ($context) : void {
                 $variableName = $node->variable->name->value;
                 if (empty($this->knownVariableNames[$variableName])) {
                     $this->knownVariableNames[$variableName] = $node->variable->name;
diff --git a/src/Validator/Rules/ValuesOfCorrectType.php b/src/Validator/Rules/ValuesOfCorrectType.php
index 2e70abff6..53b15423d 100644
--- a/src/Validator/Rules/ValuesOfCorrectType.php
+++ b/src/Validator/Rules/ValuesOfCorrectType.php
@@ -20,6 +20,7 @@
 use GraphQL\Language\AST\VariableNode;
 use GraphQL\Language\Printer;
 use GraphQL\Language\Visitor;
+use GraphQL\Language\VisitorOperation;
 use GraphQL\Type\Definition\EnumType;
 use GraphQL\Type\Definition\EnumValueDefinition;
 use GraphQL\Type\Definition\InputObjectType;
@@ -51,11 +52,11 @@ public function getVisitor(ValidationContext $context)
 
         return [
             NodeKind::FIELD        => [
-                'enter' => static function (FieldNode $node) use (&$fieldName) {
+                'enter' => static function (FieldNode $node) use (&$fieldName) : void {
                     $fieldName = $node->name->value;
                 },
             ],
-            NodeKind::NULL         => static function (NullValueNode $node) use ($context, &$fieldName) {
+            NodeKind::NULL         => static function (NullValueNode $node) use ($context, &$fieldName) : void {
                 $type = $context->getInputType();
                 if (! ($type instanceof NonNull)) {
                     return;
@@ -68,7 +69,7 @@ public function getVisitor(ValidationContext $context)
                     )
                 );
             },
-            NodeKind::LST          => function (ListValueNode $node) use ($context, &$fieldName) {
+            NodeKind::LST          => function (ListValueNode $node) use ($context, &$fieldName) : ?VisitorOperation {
                 // Note: TypeInfo will traverse into a list's item type, so look to the
                 // parent input type to check if it is a list.
                 $type = Type::getNullableType($context->getParentInputType());
@@ -77,6 +78,8 @@ public function getVisitor(ValidationContext $context)
 
                     return Visitor::skipNode();
                 }
+
+                return null;
             },
             NodeKind::OBJECT       => function (ObjectValueNode $node) use ($context, &$fieldName) {
                 // Note: TypeInfo will traverse into a list's item type, so look to the
@@ -93,7 +96,7 @@ public function getVisitor(ValidationContext $context)
                 $nodeFields   = iterator_to_array($node->fields);
                 $fieldNodeMap = array_combine(
                     array_map(
-                        static function ($field) {
+                        static function ($field) : string {
                             return $field->name->value;
                         },
                         $nodeFields
@@ -114,7 +117,7 @@ static function ($field) {
                     );
                 }
             },
-            NodeKind::OBJECT_FIELD => static function (ObjectFieldNode $node) use ($context) {
+            NodeKind::OBJECT_FIELD => static function (ObjectFieldNode $node) use ($context) : void {
                 $parentType = Type::getNamedType($context->getParentInputType());
                 /** @var ScalarType|EnumType|InputObjectType|ListOfType|NonNull $fieldType */
                 $fieldType = $context->getInputType();
@@ -137,7 +140,7 @@ static function ($field) {
                     )
                 );
             },
-            NodeKind::ENUM         => function (EnumValueNode $node) use ($context, &$fieldName) {
+            NodeKind::ENUM         => function (EnumValueNode $node) use ($context, &$fieldName) : void {
                 $type = Type::getNamedType($context->getInputType());
                 if (! $type instanceof EnumType) {
                     $this->isValidScalar($context, $node, $fieldName);
@@ -156,16 +159,16 @@ static function ($field) {
                     );
                 }
             },
-            NodeKind::INT          => function (IntValueNode $node) use ($context, &$fieldName) {
+            NodeKind::INT          => function (IntValueNode $node) use ($context, &$fieldName) : void {
                 $this->isValidScalar($context, $node, $fieldName);
             },
-            NodeKind::FLOAT        => function (FloatValueNode $node) use ($context, &$fieldName) {
+            NodeKind::FLOAT        => function (FloatValueNode $node) use ($context, &$fieldName) : void {
                 $this->isValidScalar($context, $node, $fieldName);
             },
-            NodeKind::STRING       => function (StringValueNode $node) use ($context, &$fieldName) {
+            NodeKind::STRING       => function (StringValueNode $node) use ($context, &$fieldName) : void {
                 $this->isValidScalar($context, $node, $fieldName);
             },
-            NodeKind::BOOLEAN      => function (BooleanValueNode $node) use ($context, &$fieldName) {
+            NodeKind::BOOLEAN      => function (BooleanValueNode $node) use ($context, &$fieldName) : void {
                 $this->isValidScalar($context, $node, $fieldName);
             },
         ];
diff --git a/src/Validator/Rules/VariablesAreInputTypes.php b/src/Validator/Rules/VariablesAreInputTypes.php
index 5dc27b433..a1daafff0 100644
--- a/src/Validator/Rules/VariablesAreInputTypes.php
+++ b/src/Validator/Rules/VariablesAreInputTypes.php
@@ -18,7 +18,7 @@ class VariablesAreInputTypes extends ValidationRule
     public function getVisitor(ValidationContext $context)
     {
         return [
-            NodeKind::VARIABLE_DEFINITION => static function (VariableDefinitionNode $node) use ($context) {
+            NodeKind::VARIABLE_DEFINITION => static function (VariableDefinitionNode $node) use ($context) : void {
                 $type = TypeInfo::typeFromAST($context->getSchema(), $node->type);
 
                 // If the variable type is not an input type, return an error.
diff --git a/src/Validator/Rules/VariablesInAllowedPosition.php b/src/Validator/Rules/VariablesInAllowedPosition.php
index a3932fd92..717bd6563 100644
--- a/src/Validator/Rules/VariablesInAllowedPosition.php
+++ b/src/Validator/Rules/VariablesInAllowedPosition.php
@@ -32,10 +32,10 @@ public function getVisitor(ValidationContext $context)
     {
         return [
             NodeKind::OPERATION_DEFINITION => [
-                'enter' => function () {
+                'enter' => function () : void {
                     $this->varDefMap = [];
                 },
-                'leave' => function (OperationDefinitionNode $operation) use ($context) {
+                'leave' => function (OperationDefinitionNode $operation) use ($context) : void {
                     $usages = $context->getRecursiveVariableUsages($operation);
 
                     foreach ($usages as $usage) {
@@ -68,7 +68,7 @@ public function getVisitor(ValidationContext $context)
                     }
                 },
             ],
-            NodeKind::VARIABLE_DEFINITION  => function (VariableDefinitionNode $varDefNode) {
+            NodeKind::VARIABLE_DEFINITION  => function (VariableDefinitionNode $varDefNode) : void {
                 $this->varDefMap[$varDefNode->variable->name->value] = $varDefNode;
             },
         ];
diff --git a/src/Validator/ValidationContext.php b/src/Validator/ValidationContext.php
index 67f9fbb0c..30ff56cf8 100644
--- a/src/Validator/ValidationContext.php
+++ b/src/Validator/ValidationContext.php
@@ -105,13 +105,13 @@ private function getVariableUsages(HasSelectionSet $node)
                 Visitor::visitWithTypeInfo(
                     $typeInfo,
                     [
-                        NodeKind::VARIABLE_DEFINITION => static function () {
+                        NodeKind::VARIABLE_DEFINITION => static function () : bool {
                             return false;
                         },
                         NodeKind::VARIABLE            => static function (VariableNode $variable) use (
                             &$newUsages,
                             $typeInfo
-                        ) {
+                        ) : void {
                             $newUsages[] = [
                                 'node' => $variable,
                                 'type' => $typeInfo->getInputType(),
diff --git a/tests/Type/QueryPlanTest.php b/tests/Type/QueryPlanTest.php
index 0e0e5abfa..20c6f555b 100644
--- a/tests/Type/QueryPlanTest.php
+++ b/tests/Type/QueryPlanTest.php
@@ -32,7 +32,7 @@ public function testQueryPlan() : void
 
         $author = new ObjectType([
             'name'   => 'Author',
-            'fields' => static function () use ($image, &$article) {
+            'fields' => static function () use ($image, &$article) : array {
                 return [
                     'id'            => ['type' => Type::string()],
                     'name'          => ['type' => Type::string()],
@@ -302,7 +302,7 @@ public function testQueryPlanOnInterface() : void
     {
         $petType = new InterfaceType([
             'name'   => 'Pet',
-            'fields' => static function () {
+            'fields' => static function () : array {
                 return [
                     'name' => ['type' => Type::string()],
                 ];
@@ -312,10 +312,10 @@ public function testQueryPlanOnInterface() : void
         $dogType = new ObjectType([
             'name'       => 'Dog',
             'interfaces' => [$petType],
-            'isTypeOf'   => static function ($obj) {
+            'isTypeOf'   => static function ($obj) : bool {
                 return $obj instanceof Dog;
             },
-            'fields' => static function () {
+            'fields' => static function () : array {
                 return [
                     'name'  => ['type' => Type::string()],
                     'woofs' => ['type' => Type::boolean()],
@@ -372,7 +372,7 @@ public function testQueryPlanOnInterface() : void
                     ) use (
                         &$hasCalled,
                         &$queryPlan
-) {
+                    ) : array {
                         $hasCalled = true;
                         $queryPlan = $info->lookAhead();
 
diff --git a/tests/Type/ResolveInfoTest.php b/tests/Type/ResolveInfoTest.php
index d05a44c35..000711f53 100644
--- a/tests/Type/ResolveInfoTest.php
+++ b/tests/Type/ResolveInfoTest.php
@@ -28,7 +28,7 @@ public function testFieldSelection() : void
 
         $author = new ObjectType([
             'name'   => 'Author',
-            'fields' => static function () use ($image, &$article) {
+            'fields' => static function () use ($image, &$article) : array {
                 return [
                     'id'            => ['type' => Type::string()],
                     'name'          => ['type' => Type::string()],
@@ -224,7 +224,7 @@ public function testMergedFragmentsFieldSelection() : void
 
         $author = new ObjectType([
             'name'   => 'Author',
-            'fields' => static function () use ($image, &$article) {
+            'fields' => static function () use ($image, &$article) : array {
                 return [
                     'id'            => ['type' => Type::string()],
                     'name'          => ['type' => Type::string()],
diff --git a/tests/Type/SchemaTest.php b/tests/Type/SchemaTest.php
index 37075ec94..9b401a49f 100644
--- a/tests/Type/SchemaTest.php
+++ b/tests/Type/SchemaTest.php
@@ -46,7 +46,7 @@ public function setUp() : void
             'fields'     => [
                 'fieldName' => [
                     'type'    => Type::string(),
-                    'resolve' => static function () {
+                    'resolve' => static function () : string {
                         return '';
                     },
                 ],
@@ -90,7 +90,7 @@ public function setUp() : void
                 'fields' => [
                     'getObject' => [
                         'type'    => $this->interfaceType,
-                        'resolve' => static function () {
+                        'resolve' => static function () : array {
                             return [];
                         },
                     ],
diff --git a/tests/Type/StandardTypesTest.php b/tests/Type/StandardTypesTest.php
index 00e182079..1f1989142 100644
--- a/tests/Type/StandardTypesTest.php
+++ b/tests/Type/StandardTypesTest.php
@@ -121,11 +121,11 @@ private function createCustomScalarType($name)
     {
         return new CustomScalarType([
             'name' => $name,
-            'serialize' => static function () {
+            'serialize' => static function () : void {
             },
-            'parseValue' => static function () {
+            'parseValue' => static function () : void {
             },
-            'parseLiteral' => static function () {
+            'parseLiteral' => static function () : void {
             },
         ]);
     }
diff --git a/tests/Type/TypeLoaderTest.php b/tests/Type/TypeLoaderTest.php
index ab87c70aa..ddf8a2137 100644
--- a/tests/Type/TypeLoaderTest.php
+++ b/tests/Type/TypeLoaderTest.php
@@ -54,20 +54,20 @@ public function setUp() : void
 
         $this->node = new InterfaceType([
             'name'        => 'Node',
-            'fields'      => function () {
+            'fields'      => function () : array {
                 $this->calls[] = 'Node.fields';
 
                 return [
                     'id' => Type::string(),
                 ];
             },
-            'resolveType' => static function () {
+            'resolveType' => static function () : void {
             },
         ]);
 
         $this->content = new InterfaceType([
             'name'        => 'Content',
-            'fields'      => function () {
+            'fields'      => function () : array {
                 $this->calls[] = 'Content.fields';
 
                 return [
@@ -75,7 +75,7 @@ public function setUp() : void
                     'body'  => Type::string(),
                 ];
             },
-            'resolveType' => static function () {
+            'resolveType' => static function () : void {
             },
         ]);
 
@@ -85,7 +85,7 @@ public function setUp() : void
                 $this->node,
                 $this->content,
             ],
-            'fields'     => function () {
+            'fields'     => function () : array {
                 $this->calls[] = 'BlogStory.fields';
 
                 return [
@@ -98,7 +98,7 @@ public function setUp() : void
 
         $this->query = new ObjectType([
             'name'   => 'Query',
-            'fields' => function () {
+            'fields' => function () : array {
                 $this->calls[] = 'Query.fields';
 
                 return [
@@ -110,7 +110,7 @@ public function setUp() : void
 
         $this->mutation = new ObjectType([
             'name'   => 'Mutation',
-            'fields' => function () {
+            'fields' => function () : array {
                 $this->calls[] = 'Mutation.fields';
 
                 return [
@@ -158,7 +158,7 @@ public function testSchemaAcceptsTypeLoader() : void
                 'name'   => 'Query',
                 'fields' => ['a' => Type::string()],
             ]),
-            'typeLoader' => static function () {
+            'typeLoader' => static function () : void {
             },
         ]);
     }
@@ -259,7 +259,7 @@ public function testFailsOnNonExistentType() : void
     {
         $schema = new Schema([
             'query'      => $this->query,
-            'typeLoader' => static function () {
+            'typeLoader' => static function () : void {
             },
         ]);
 
@@ -273,7 +273,7 @@ public function testFailsOnNonType() : void
     {
         $schema = new Schema([
             'query'      => $this->query,
-            'typeLoader' => static function () {
+            'typeLoader' => static function () : stdClass {
                 return new stdClass();
             },
         ]);
@@ -288,7 +288,7 @@ public function testFailsOnInvalidLoad() : void
     {
         $schema = new Schema([
             'query'      => $this->query,
-            'typeLoader' => function () {
+            'typeLoader' => function () : InterfaceType {
                 return $this->content;
             },
         ]);
@@ -303,7 +303,7 @@ public function testPassesThroughAnExceptionInLoader() : void
     {
         $schema = new Schema([
             'query'      => $this->query,
-            'typeLoader' => static function () {
+            'typeLoader' => static function () : void {
                 throw new Exception('This is the exception we are looking for');
             },
         ]);
diff --git a/tests/Type/ValidationTest.php b/tests/Type/ValidationTest.php
index ca21338c2..f374643e7 100644
--- a/tests/Type/ValidationTest.php
+++ b/tests/Type/ValidationTest.php
@@ -13,6 +13,8 @@
 use GraphQL\Type\Definition\EnumType;
 use GraphQL\Type\Definition\InputObjectType;
 use GraphQL\Type\Definition\InterfaceType;
+use GraphQL\Type\Definition\ListOfType;
+use GraphQL\Type\Definition\NonNull;
 use GraphQL\Type\Definition\ObjectType;
 use GraphQL\Type\Definition\ScalarType;
 use GraphQL\Type\Definition\Type;
@@ -65,27 +67,27 @@ public function setUp() : void
 
         $this->SomeScalarType = new CustomScalarType([
             'name'         => 'SomeScalar',
-            'serialize'    => static function () {
+            'serialize'    => static function () : void {
             },
-            'parseValue'   => static function () {
+            'parseValue'   => static function () : void {
             },
-            'parseLiteral' => static function () {
+            'parseLiteral' => static function () : void {
             },
         ]);
 
         $this->SomeInterfaceType = new InterfaceType([
             'name'   => 'SomeInterface',
-            'fields' => function () {
+            'fields' => function () : array {
                 return ['f' => ['type' => $this->SomeObjectType]];
             },
         ]);
 
         $this->SomeObjectType = new ObjectType([
             'name'       => 'SomeObject',
-            'fields'     => function () {
+            'fields'     => function () : array {
                 return ['f' => ['type' => $this->SomeObjectType]];
             },
-            'interfaces' => function () {
+            'interfaces' => function () : array {
                 return [$this->SomeInterfaceType];
             },
         ]);
@@ -144,19 +146,19 @@ private function withModifiers($types)
             $types,
             Utils::map(
                 $types,
-                static function ($type) {
+                static function ($type) : ListOfType {
                     return Type::listOf($type);
                 }
             ),
             Utils::map(
                 $types,
-                static function ($type) {
+                static function ($type) : NonNull {
                     return Type::nonNull($type);
                 }
             ),
             Utils::map(
                 $types,
-                static function ($type) {
+                static function ($type) : NonNull {
                     return Type::nonNull(Type::listOf($type));
                 }
             )
@@ -173,19 +175,19 @@ public function testRejectsTypesWithoutNames() : void
     {
         $this->assertEachCallableThrows(
             [
-                static function () {
+                static function () : ObjectType {
                     return new ObjectType([]);
                 },
-                static function () {
+                static function () : EnumType {
                     return new EnumType([]);
                 },
-                static function () {
+                static function () : InputObjectType {
                     return new InputObjectType([]);
                 },
-                static function () {
+                static function () : UnionType {
                     return new UnionType([]);
                 },
-                static function () {
+                static function () : InterfaceType {
                     return new InterfaceType([]);
                 },
             ],
@@ -335,7 +337,7 @@ public function testRejectsASchemaWithoutAQueryType() : void
 
     private function formatLocations(Error $error)
     {
-        return Utils::map($error->getLocations(), static function (SourceLocation $loc) {
+        return Utils::map($error->getLocations(), static function (SourceLocation $loc) : array {
             return ['line' => $loc->line, 'column' => $loc->column];
         });
     }
@@ -348,7 +350,7 @@ private function formatLocations(Error $error)
      */
     private function formatErrors(array $errors, $withLocation = true)
     {
-        return Utils::map($errors, function (Error $error) use ($withLocation) {
+        return Utils::map($errors, function (Error $error) use ($withLocation) : array {
             if (! $withLocation) {
                 return [ 'message' => $error->getMessage() ];
             }
@@ -641,7 +643,7 @@ public function testRejectsAnObjectTypeWithMissingFields() : void
         $manualSchema2 = $this->schemaWithFieldType(
             new ObjectType([
                 'name'   => 'IncompleteObject',
-                'fields' => static function () {
+                'fields' => static function () : array {
                     return [];
                 },
             ])
@@ -2355,7 +2357,7 @@ interface AnotherInterface {
     public function testRejectsDifferentInstancesOfTheSameType() : void
     {
         // Invalid: always creates new instance vs returning one from registry
-        $typeLoader = static function ($name) {
+        $typeLoader = static function ($name) : ?ObjectType {
             switch ($name) {
                 case 'Query':
                     return new ObjectType([
diff --git a/tests/Utils/ExtractTypesTest.php b/tests/Utils/ExtractTypesTest.php
index 757c8ce8a..be2de6dfe 100644
--- a/tests/Utils/ExtractTypesTest.php
+++ b/tests/Utils/ExtractTypesTest.php
@@ -65,7 +65,7 @@ public function setUp() : void
 
         $this->content = new InterfaceType([
             'name'   => 'Content',
-            'fields' => function () {
+            'fields' => function () : array {
                 return [
                     'title'      => Type::string(),
                     'body'       => Type::string(),
@@ -82,7 +82,7 @@ public function setUp() : void
                 $this->node,
                 $this->content,
             ],
-            'fields'     => function () {
+            'fields'     => function () : array {
                 return [
                     $this->node->getField('id'),
                     $this->content->getField('title'),
@@ -100,7 +100,7 @@ public function setUp() : void
                 $this->node,
                 $this->content,
             ],
-            'fields'     => function () {
+            'fields'     => function () : array {
                 return [
                     'id'         => $this->node->getField('id'),
                     'title'      => $this->content->getField('title'),
@@ -119,7 +119,7 @@ public function setUp() : void
                 $this->node,
                 $this->content,
             ],
-            'fields'     => function () {
+            'fields'     => function () : array {
                 return [
                     'id'          => $this->node->getField('id'),
                     'title'       => $this->content->getField('title'),
@@ -145,7 +145,7 @@ public function setUp() : void
             'interfaces' => [
                 $this->node,
             ],
-            'fields'     => function () {
+            'fields'     => function () : array {
                 return [
                     'id'      => $this->node->getField('id'),
                     'author'  => $this->user,
@@ -162,7 +162,7 @@ public function setUp() : void
             'interfaces' => [
                 $this->node,
             ],
-            'fields'     => function () {
+            'fields'     => function () : array {
                 return [
                     'id'   => $this->node->getField('id'),
                     'name' => Type::string(),
@@ -175,7 +175,7 @@ public function setUp() : void
             'interfaces' => [
                 $this->node,
             ],
-            'fields'     => function () {
+            'fields'     => function () : array {
                 return [
                     'id'   => $this->node->getField('id'),
                     'name' => Type::string(),
diff --git a/tests/Utils/MixedStoreTest.php b/tests/Utils/MixedStoreTest.php
index fd04d2e0c..e23460daf 100644
--- a/tests/Utils/MixedStoreTest.php
+++ b/tests/Utils/MixedStoreTest.php
@@ -38,7 +38,7 @@ public function getPossibleValues()
             'a',
             [],
             new stdClass(),
-            static function () {
+            static function () : void {
             },
             new MixedStore(),
         ];
@@ -124,7 +124,7 @@ public function testAcceptsObjectKeys() : void
             $this->assertAcceptsKeyValue(new stdClass(), $value);
             $this->assertAcceptsKeyValue(new MixedStore(), $value);
             $this->assertAcceptsKeyValue(
-                static function () {
+                static function () : void {
                 },
                 $value
             );
diff --git a/tests/Utils/SchemaExtenderTest.php b/tests/Utils/SchemaExtenderTest.php
index 61cdb18a0..a7d9729bc 100644
--- a/tests/Utils/SchemaExtenderTest.php
+++ b/tests/Utils/SchemaExtenderTest.php
@@ -68,7 +68,7 @@ public function setUp() : void
 
         $SomeInterfaceType = new InterfaceType([
             'name' => 'SomeInterface',
-            'fields' => static function () use (&$SomeInterfaceType) {
+            'fields' => static function () use (&$SomeInterfaceType) : array {
                 return [
                     'name' => [ 'type' => Type::string()],
                     'some' => [ 'type' => $SomeInterfaceType],
@@ -79,7 +79,7 @@ public function setUp() : void
         $FooType = new ObjectType([
             'name' => 'Foo',
             'interfaces' => [$SomeInterfaceType],
-            'fields' => static function () use ($SomeInterfaceType, &$FooType) {
+            'fields' => static function () use ($SomeInterfaceType, &$FooType) : array {
                 return [
                     'name' => [ 'type' => Type::string() ],
                     'some' => [ 'type' => $SomeInterfaceType ],
@@ -184,7 +184,7 @@ public function setUp() : void
 
         $testSchemaAst = Parser::parse(SchemaPrinter::doPrint($this->testSchema));
 
-        $this->testSchemaDefinitions = array_map(static function ($node) {
+        $this->testSchemaDefinitions = array_map(static function ($node) : string {
             return Printer::doPrint($node);
         }, iterator_to_array($testSchemaAst->definitions->getIterator()));
 
@@ -1156,19 +1156,19 @@ public function testMayExtendMutationsAndSubscriptions()
         $mutationSchema = new Schema([
             'query' => new ObjectType([
                 'name' => 'Query',
-                'fields' => static function () {
+                'fields' => static function () : array {
                     return [ 'queryField' => [ 'type' => Type::string() ] ];
                 },
             ]),
             'mutation' => new ObjectType([
                 'name' => 'Mutation',
-                'fields' => static function () {
+                'fields' => static function () : array {
                     return [ 'mutationField' => ['type' => Type::string() ] ];
                 },
             ]),
             'subscription' => new ObjectType([
                 'name' => 'Subscription',
-                'fields' => static function () {
+                'fields' => static function () : array {
                     return ['subscriptionField' => ['type' => Type::string()]];
                 },
             ]),
@@ -1357,7 +1357,7 @@ public function testDoesNotAllowReplacingACustomDirective()
      */
     public function testDoesNotAllowReplacingAnExistingType()
     {
-        $existingTypeError = static function ($type) {
+        $existingTypeError = static function ($type) : string {
             return 'Type "' . $type . '" already exists in the schema. It cannot also be defined in this type definition.';
         };
 
@@ -1433,7 +1433,7 @@ enum SomeEnum
      */
     public function testDoesNotAllowReplacingAnExistingField()
     {
-        $existingFieldError = static function (string $type, string $field) {
+        $existingFieldError = static function (string $type, string $field) : string {
             return 'Field "' . $type . '.' . $field . '" already exists in the schema. It cannot also be defined in this type extension.';
         };
 
@@ -1786,7 +1786,7 @@ public function testSchemaExtensionASTAreAvailableFromSchemaObject()
             '),
             implode(
                 "\n",
-                array_map(static function ($node) {
+                array_map(static function ($node) : string {
                     return Printer::doPrint($node) . "\n";
                 }, $nodes)
             )
@@ -1879,7 +1879,7 @@ public function testOriginalResolversArePreserved()
             'fields' => [
                 'hello' => [
                     'type' => Type::string(),
-                    'resolve' => static function () {
+                    'resolve' => static function () : string {
                         return 'Hello World!';
                     },
                 ],
diff --git a/tests/Validator/OverlappingFieldsCanBeMergedTest.php b/tests/Validator/OverlappingFieldsCanBeMergedTest.php
index e66ee71f1..9b3879431 100644
--- a/tests/Validator/OverlappingFieldsCanBeMergedTest.php
+++ b/tests/Validator/OverlappingFieldsCanBeMergedTest.php
@@ -704,7 +704,7 @@ private function getSchema()
 
         $SomeBox = new InterfaceType([
             'name'   => 'SomeBox',
-            'fields' => static function () use (&$SomeBox) {
+            'fields' => static function () use (&$SomeBox) : array {
                 return [
                     'deepBox'        => ['type' => $SomeBox],
                     'unrelatedField' => ['type' => Type::string()],
@@ -715,7 +715,7 @@ private function getSchema()
         $StringBox = new ObjectType([
             'name'       => 'StringBox',
             'interfaces' => [$SomeBox],
-            'fields'     => static function () use (&$StringBox, &$IntBox) {
+            'fields'     => static function () use (&$StringBox, &$IntBox) : array {
                 return [
                     'scalar'         => ['type' => Type::string()],
                     'deepBox'        => ['type' => $StringBox],
@@ -730,7 +730,7 @@ private function getSchema()
         $IntBox = new ObjectType([
             'name'       => 'IntBox',
             'interfaces' => [$SomeBox],
-            'fields'     => static function () use (&$StringBox, &$IntBox) {
+            'fields'     => static function () use (&$StringBox, &$IntBox) : array {
                 return [
                     'scalar'         => ['type' => Type::int()],
                     'deepBox'        => ['type' => $IntBox],
diff --git a/tests/Validator/QueryComplexityTest.php b/tests/Validator/QueryComplexityTest.php
index d47c9c5b1..0a14fb966 100644
--- a/tests/Validator/QueryComplexityTest.php
+++ b/tests/Validator/QueryComplexityTest.php
@@ -199,10 +199,10 @@ public function testSkippedWhenThereAreOtherValidationErrors() : void
         $reportedError = new Error('OtherValidatorError');
         $otherRule     = new CustomValidationRule(
             'otherRule',
-            static function (ValidationContext $context) use ($reportedError) {
+            static function (ValidationContext $context) use ($reportedError) : array {
                 return [
                     NodeKind::OPERATION_DEFINITION => [
-                        'leave' => static function () use ($context, $reportedError) {
+                        'leave' => static function () use ($context, $reportedError) : void {
                             $context->reportError($reportedError);
                         },
                     ],
diff --git a/tests/Validator/ValidatorTestCase.php b/tests/Validator/ValidatorTestCase.php
index 641039835..0944f0e5c 100644
--- a/tests/Validator/ValidatorTestCase.php
+++ b/tests/Validator/ValidatorTestCase.php
@@ -64,7 +64,7 @@ public static function getTestSchema()
 
         $Canine = new InterfaceType([
             'name'   => 'Canine',
-            'fields' => static function () {
+            'fields' => static function () : array {
                 return [
                     'name' => [
                         'type' => Type::string(),
@@ -111,7 +111,7 @@ public static function getTestSchema()
 
         $Cat = new ObjectType([
             'name'       => 'Cat',
-            'fields'     => static function () use (&$FurColor) {
+            'fields'     => static function () use (&$FurColor) : array {
                 return [
                     'name'       => [
                         'type' => Type::string(),
@@ -142,7 +142,7 @@ public static function getTestSchema()
         $Human = new ObjectType([
             'name'       => 'Human',
             'interfaces' => [$Being, $Intelligent],
-            'fields'     => static function () use (&$Human, $Pet) {
+            'fields'     => static function () use (&$Human, $Pet) : array {
                 return [
                     'name'      => [
                         'type' => Type::string(),
@@ -300,10 +300,10 @@ public static function getTestSchema()
             'serialize'    => static function ($value) {
                 return $value;
             },
-            'parseLiteral' => static function ($node) {
+            'parseLiteral' => static function ($node) : void {
                 throw new Exception('Invalid scalar is always invalid: ' . $node->value);
             },
-            'parseValue'   => static function ($node) {
+            'parseValue'   => static function ($node) : void {
                 throw new Exception('Invalid scalar is always invalid: ' . $node);
             },
         ]);