fix: Type case with only reserved fields is not composite inline fragment

Tests/ApolloCodegenInternalTestHelpers/IRBuilder+Mocking.swift

@@ -71,7 +71,8 @@ extension IR.NamedFragment {
           inclusionConditions: nil,
           givenAllTypesInSchema: .init([type], schemaRootTypes: .mock())
         )
-      ]
+      ],
+      isUserDefined: true
     )
     let rootEntityField = IR.EntityField.init(
       rootField,
@@ -110,7 +111,8 @@ extension IR.Operation {
         forType: .mock(),
         inclusionConditions: nil,
         givenAllTypesInSchema: .init([], schemaRootTypes: .mock()))
-      ]
+      ],
+      isUserDefined: true
     )
     let rootField = IR.EntityField(
       .mock(),

Tests/ApolloCodegenTests/CodeGeneration/Templates/FragmentTemplateTests.swift

@@ -303,8 +303,6 @@ class FragmentTemplateTests: XCTestCase {
       init(_dataDict: DataDict) { __data = _dataDict }
 
       static var __parentType: ApolloAPI.ParentType { TestSchema.Objects.Query }
-      static var __selections: [ApolloAPI.Selection] { [
-      ] }
     }
 
     """

Tests/ApolloCodegenTests/CodeGeneration/Templates/SelectionSet/SelectionSetTemplateTests.swift

@@ -1447,6 +1447,57 @@
     expect(actual).to(equalLineByLine(expected, atLine: 7, ignoringExtraLines: true))
   }
 
+  // Related to https://github.com/apollographql/apollo-ios/issues/3326
+  func test__render_selections__givenTypeCaseWithOnlyReservedField_doesNotRenderSelections() async throws {
+    // given
+    schemaSDL = """
+    type Query {
+      allAnimals: [Animal]
+    }
+
+    union Animal = AnimalObject | AnimalError
+
+    type AnimalObject {
+      species: String!
+    }
+
+    type AnimalError {
+      code: Int!
+    }
+    """
+
+    document = """
+    query TestOperation {
+      allAnimals {
+        ... on AnimalObject {
+          __typename
+        }
+      }
+    }
+    """
+
+    let expected = """
+    public struct AsAnimalObject: TestSchema.InlineFragment {
+      public let __data: DataDict
+      public init(_dataDict: DataDict) { __data = _dataDict }
+
+      public typealias RootEntityType = TestOperationQuery.Data.AllAnimal
+      public static var __parentType: ApolloAPI.ParentType { TestSchema.Objects.AnimalObject }
+    }
+    """
+
+    // when
+    try await buildSubjectAndOperation()
+    let allAnimals_asAnimalObject = try XCTUnwrap(
+      operation[field: "query"]?[field: "allAnimals"]?[as: "AnimalObject"]
+    )
+
+    let actual = subject.test_render(inlineFragment: allAnimals_asAnimalObject.computed)
+
+    // then
+    expect(actual).to(equalLineByLine(expected, atLine: 2, ignoringExtraLines: true))
+  }
+
   // MARK: Selections - Fragments
 
   func test__render_selections__givenFragments_rendersFragmentSelections() async throws {
@@ -1716,11 +1767,11 @@
     ))
   }
 
   #warning("need more tests here - same test cases as IRRootFieldBuilderTests")
 
   // MARK: Selections - Deferred Named Fragment
 
   #warning("need more tests here - same test cases as IRRootFieldBuilderTests")
 
   // MARK: Selections - Include/Skip
 
@@ -4664,11 +4715,11 @@
     ))
   }
 
   #warning("need more tests here - same test cases as IRRootFieldBuilderTests")
 
   // MARK: Field Accessors - Deferred Named Fragments
 
   #warning("need more tests here - same test cases as IRRootFieldBuilderTests")
 
   // MARK: - Inline Fragment Accessors
 
@@ -5522,11 +5573,11 @@
     ))
   }
 
   #warning("need more tests here - same test cases as IRRootFieldBuilderTests")
 
   // MARK: Fragment Accessors - Deferred Named Fragment
 
   #warning("need more tests here - same test cases as IRRootFieldBuilderTests")
 
   // MARK: - Nested Selection Sets
 
@@ -6952,6 +7003,54 @@
       .to(equalLineByLine(predator_expected, atLine: 21, ignoringExtraLines: true))
   }
 
+// Related to https://github.com/apollographql/apollo-ios/issues/3326
+  func test__render_nestedSelectionSet__givenInlineFragmentWithOnlyReservedField_doesNotRenderAsCompositeInlineFragment() async throws {
+    // given
+    schemaSDL = """
+    type Query {
+      allAnimals: [Animal]
+    }
+
+    union Animal = AnimalObject | AnimalError
+
+    type AnimalObject {
+      species: String!
+    }
+
+    type AnimalError {
+      code: Int!
+    }
+    """
+
+    document = """
+    query TestOperation {
+      allAnimals {
+        ... on AnimalObject {
+          __typename
+        }
+      }
+    }
+    """
+
+    let expected = """
+      public var asAnimalObject: AsAnimalObject? { _asInlineFragment() }
+
+      /// AllAnimal.AsAnimalObject
+      public struct AsAnimalObject: TestSchema.InlineFragment {
+    """
+
+    // when
+    try await buildSubjectAndOperation()
+    let allAnimals = try XCTUnwrap(
+      operation[field: "query"]?[field: "allAnimals"]?.selectionSet
+    )
+
+    let actual = subject.test_render(childEntity: allAnimals.computed)
+
+    // then
+    expect(actual).to(equalLineByLine(expected, atLine: 12, ignoringExtraLines: true))
+  }
+
   // MARK: Nested Selection Sets - Reserved Keywords + Special Names
 
   func test__render_nestedSelectionSet__givenEntityFieldWithSwiftKeywordAndApolloReservedTypeNames_rendersSelectionSetWithNameSuffixed() async throws {
@@ -7532,7 +7631,7 @@
     ))
   }
 
   #warning("need more tests here - same test cases as IRRootFieldBuilderTests (inline fragment only")
 
   // MARK: - Documentation Tests
 

Tests/CodegenIntegrationTests/Tests/3326-typeCaseWithOnlyReservedFieldSelectionIsNotCompositeInlineFragment/README.MD

@@ -0,0 +1,11 @@
+# Overview
+
+When a type case selection consisted of only GraphQL reserved fields, such as `__typename` the selection was not being recognized and a composite inline fragment was generated. This resulted in empty `__selections` and `__mergedSources` lists. This in turn caused the `_asInlineFragment` accessor to behave incorrectly because it checks whether the definitions contained within `__mergedSources` are a match for the returned type. The empty list always matches and `_asInlineFragment` incorrectly matches the returned type.
+
+We should not be generating the inline fragment with `CompositeInlineFragment` conformance, which will then not generate the empty `__mergedSources` list. Codegen will also no longer generate empty `__selections` lists and this integration test was created to ensure that the generated model still conforms to `SelectionSet` through the extensions default implementation of `__selections`.
+
+## Reference Issue: https://github.com/apollographql/apollo-ios/issues/3326
+
+## Solution
+
+The `TypeInfo` for a selection now has a property indicating whether it was defined by the user or formed through a combination of external selections, i.e.: composite. This allows the root field builder to definitively specify under which conditions a composite inline fragment should be generated.

Tests/CodegenIntegrationTests/Tests/3326-typeCaseWithOnlyReservedFieldSelectionIsNotCompositeInlineFragment/operation.graphql

@@ -0,0 +1,7 @@
+  query TestOperation {
+  allAnimals {
+    ... on AnimalObject {
+      __typename
+    }
+  }
+}

Tests/CodegenIntegrationTests/Tests/3326-typeCaseWithOnlyReservedFieldSelectionIsNotCompositeInlineFragment/schema.graphqls

@@ -0,0 +1,13 @@
+type Query {
+  allAnimals: [Animal]
+}
+
+union Animal = AnimalObject | AnimalError
+
+type AnimalObject {
+  species: String!
+}
+
+type AnimalError {
+  code: Int!
+}

apollo-ios-codegen/Sources/ApolloCodegenLib/Templates/SelectionSetTemplate.swift

@@ -121,7 +121,7 @@ struct SelectionSetTemplate {
       \(SelectionSetNameDocumentation(inlineFragment))
       \(renderAccessControl())\
       struct \(inlineFragment.renderedTypeName): \(SelectionSetType(asInlineFragment: true))\
-      \(if: inlineFragment.isCompositeSelectionSet, ", \(config.ApolloAPITargetName).CompositeInlineFragment")\
+      \(if: inlineFragment.isCompositeInlineFragment, ", \(config.ApolloAPITargetName).CompositeInlineFragment")\
       \(if: inlineFragment.isDeferred, ", \(config.ApolloAPITargetName).Deferrable")\
        {
         \(BodyTemplate(context))
@@ -274,18 +274,25 @@ struct SelectionSetTemplate {
     var deprecatedArguments: [DeprecatedArgument]? =
       config.options.warningsOnDeprecatedUsage == .include ? [] : nil
 
-    let selectionsTemplate = TemplateString(
-      """
-      \(renderAccessControl())\
-      static var __selections: [\(config.ApolloAPITargetName).Selection] { [
-        \(if: shouldIncludeTypenameSelection(for: scope), ".field(\"__typename\", String.self),")
-        \(renderedSelections(groupedSelections.unconditionalSelections, &deprecatedArguments), terminator: ",")
-        \(groupedSelections.inclusionConditionGroups.map {
-        renderedConditionalSelectionGroup($0, $1, in: scope, &deprecatedArguments)
-      }, terminator: ",")
-      ] }
-      """
-    )
+    let shouldIncludeTypenameSelection = shouldIncludeTypenameSelection(for: scope)
+    let selectionsTemplate: TemplateString
+
+    if !groupedSelections.isEmpty || shouldIncludeTypenameSelection {
+      selectionsTemplate = TemplateString("""
+        \(renderAccessControl())\
+        static var __selections: [\(config.ApolloAPITargetName).Selection] { [
+          \(if: shouldIncludeTypenameSelection, ".field(\"__typename\", String.self),")
+          \(renderedSelections(groupedSelections.unconditionalSelections, &deprecatedArguments), terminator: ",")
+          \(groupedSelections.inclusionConditionGroups.map {
+          renderedConditionalSelectionGroup($0, $1, in: scope, &deprecatedArguments)
+        }, terminator: ",")
+        ] }
+        """
+      )
+    } else {
+      selectionsTemplate = ""
+    }
+
     return """
       \(if: deprecatedArguments != nil && !deprecatedArguments.unsafelyUnwrapped.isEmpty, """
       \(deprecatedArguments.unsafelyUnwrapped.map { """
@@ -752,12 +759,8 @@ private class SelectionSetNameCache {
 
 extension IR.ComputedSelectionSet {
 
-  fileprivate var isCompositeSelectionSet: Bool {
-    return direct?.isEmpty ?? true
-  }
-
   fileprivate var isCompositeInlineFragment: Bool {
-    return !self.isEntityRoot && isCompositeSelectionSet
+    return !self.isEntityRoot && !self.isUserDefined && (direct?.isEmpty ?? true)
   }
 
   fileprivate var shouldBeRendered: Bool {

apollo-ios-codegen/Sources/IR/IR+ComputedSelectionSet.swift

@@ -104,7 +104,8 @@ extension ComputedSelectionSet {
     private func createShallowlyMergedNestedEntityField(from field: EntityField) -> EntityField {
       let typeInfo = SelectionSet.TypeInfo(
         entity: entityStorage.entity(for: field.underlyingField, on: typeInfo.entity),
-        scopePath: self.typeInfo.scopePath.appending(field.selectionSet.typeInfo.scope)
+        scopePath: self.typeInfo.scopePath.appending(field.selectionSet.typeInfo.scope),
+        isUserDefined: false
       )
 
       let newSelectionSet = SelectionSet(
@@ -149,7 +150,8 @@ extension ComputedSelectionSet {
 
       let typeInfo = SelectionSet.TypeInfo(
         entity: self.typeInfo.entity,
-        scopePath: self.typeInfo.scopePath.mutatingLast { $0.appending(condition) }
+        scopePath: self.typeInfo.scopePath.mutatingLast { $0.appending(condition) },
+        isUserDefined: false
       )
 
       let selectionSet = SelectionSet(

apollo-ios-codegen/Sources/IR/IR+DirectSelections.swift

@@ -85,7 +85,8 @@ public class DirectSelections: Equatable, CustomDebugStringConvertible {
 
     let typeInfo = SelectionSet.TypeInfo(
       entity: existingField.entity,
-      scopePath: wrapperScope
+      scopePath: wrapperScope,
+      isUserDefined: true
     )
 
     let selectionSet = SelectionSet(
@@ -111,7 +112,8 @@ public class DirectSelections: Equatable, CustomDebugStringConvertible {
         entity: newField.entity,
         scopePath: wrapperField.selectionSet.scopePath.mutatingLast {
           $0.appending(newFieldConditions)
-        }
+        },
+        isUserDefined: true
       )
 
       let newFieldSelectionSet = SelectionSet(
@@ -209,6 +211,10 @@ public class DirectSelections: Equatable, CustomDebugStringConvertible {
     public private(set) var inclusionConditionGroups:
     OrderedDictionary<AnyOf<InclusionConditions>, DirectSelections.ReadOnly> = [:]
 
+    public var isEmpty: Bool {
+      unconditionalSelections.isEmpty && inclusionConditionGroups.isEmpty
+    }
+
     init(_ directSelections: DirectSelections.ReadOnly) {
       for selection in directSelections.fields {
         if let condition = selection.value.inclusionConditions {

apollo-ios-codegen/Sources/IR/IR+RootFieldBuilder.swift

@@ -74,7 +74,8 @@ class RootFieldBuilder {
   ) async -> SelectionSet {
     let typeInfo = SelectionSet.TypeInfo(
       entity: entity,
-      scopePath: scopePath
+      scopePath: scopePath,
+      isUserDefined: true
     )
 
     var directSelections: DirectSelections? = nil
@@ -420,7 +421,8 @@ class RootFieldBuilder {
 
     let typeInfo = SelectionSet.TypeInfo(
       entity: parentTypeInfo.entity,
-      scopePath: scopePath
+      scopePath: scopePath,
+      isUserDefined: true
     )
 
     let fragmentSpread = NamedFragmentSpread(

apollo-ios-codegen/Sources/IR/IR+SelectionSet.swift

@@ -15,6 +15,13 @@ public class SelectionSet: Hashable, CustomDebugStringConvertible {
     /// The selection set's `scope` is the last element in the list.
     public let scopePath: LinkedList<ScopeDescriptor>
 
+    /// Indicates if the `SelectionSet` was created directly due to a selection set in the user defined `.graphql` definition file.
+    ///
+    /// If `false`, the selection set was artificially created by the IR. Currently, the only reason for this is a `CompositeInlineFragment` created during calculation of merged selections for field merging.
+    public var isUserDefined: Bool
+
+    // MARK: - Computed Properties
+
     /// Describes all of the types and inclusion conditions the selection set matches.
     /// Derived from all the selection set's parents.
     public var scope: ScopeDescriptor { scopePath.last.value }
@@ -28,6 +35,7 @@ public class SelectionSet: Hashable, CustomDebugStringConvertible {
     public var deferCondition: CompilationResult.DeferCondition? {
       scope.scopePath.last.value.deferCondition
     }
+
     public var isDeferred: Bool { deferCondition != nil }
 
     /// Indicates if the `SelectionSet` represents a root selection set.
@@ -38,10 +46,12 @@ public class SelectionSet: Hashable, CustomDebugStringConvertible {
 
     init(
       entity: Entity,
-      scopePath: LinkedList<ScopeDescriptor>
+      scopePath: LinkedList<ScopeDescriptor>,
+      isUserDefined: Bool
     ) {
       self.entity = entity
       self.scopePath = scopePath
+      self.isUserDefined = isUserDefined
     }
 
     public static func == (lhs: TypeInfo, rhs: TypeInfo) -> Bool {