Introduce checkElementTypeOfArrayOrString for downlevel for..of type …

…checking

src/compiler/checker.ts

@@ -1869,7 +1869,11 @@ module ts {
                 return anyType;
             }
             if (declaration.parent.parent.kind === SyntaxKind.ForOfStatement) {
-                return getTypeForVariableDeclarationInForOfStatement(<ForOfStatement>declaration.parent.parent);
+                // checkRightHandSideOfForOf will return undefined if the for-of expression type was
+                // missing properties/signatures required to get its iteratedType (like
+                // [Symbol.iterator] or next). This may be because we accessed properties from anyType,
+                // or it may have led to an error inside getIteratedType.
+                return checkRightHandSideOfForOf((<ForOfStatement>declaration.parent.parent).expression) || anyType;
             }
             if (isBindingPattern(declaration.parent)) {
                 return getTypeForBindingElement(<BindingElement>declaration);
@@ -8781,16 +8785,15 @@ module ts {
 
             // Check the LHS and RHS
             // If the LHS is a declaration, just check it as a variable declaration, which will in turn check the RHS
-            // via getTypeForVariableDeclarationInForOfStatement.
+            // via checkRightHandSideOfForOf.
             // If the LHS is an expression, check the LHS, as a destructuring assignment or as a reference.
             // Then check that the RHS is assignable to it.
             if (node.initializer.kind === SyntaxKind.VariableDeclarationList) {
                 checkForInOrForOfVariableDeclaration(node);
             }
             else {
                 var varExpr = <Expression>node.initializer;
-                var rightType = checkExpression(node.expression);
-                var iteratedType = checkIteratedType(rightType, node.expression);
+                var iteratedType = checkRightHandSideOfForOf(node.expression);
 
                 // There may be a destructuring assignment on the left side
                 if (varExpr.kind === SyntaxKind.ArrayLiteralExpression || varExpr.kind === SyntaxKind.ObjectLiteralExpression) {
@@ -8872,18 +8875,11 @@ module ts {
             }
         }
 
-        function getTypeForVariableDeclarationInForOfStatement(forOfStatement: ForOfStatement): Type {
-            // Temporarily return 'any' below ES6
-            if (languageVersion < ScriptTarget.ES6) {
-                return anyType;
-            }
-
-            // iteratedType will be undefined if the for-of expression type was missing properties/signatures
-            // required to get its iteratedType (like [Symbol.iterator] or next). This may be
-            // because we accessed properties from anyType, or it may have led to an error inside
-            // getIteratedType.
-            var expressionType = getTypeOfExpression(forOfStatement.expression);
-            return checkIteratedType(expressionType, forOfStatement.expression) || anyType;
+        function checkRightHandSideOfForOf(rhsExpression: Expression): Type {
+            var expressionType = getTypeOfExpression(rhsExpression);
+            return languageVersion >= ScriptTarget.ES6
+                ? checkIteratedType(expressionType, rhsExpression)
+                : checkElementTypeOfArrayOrString(expressionType, rhsExpression);
         }
 
         /**
@@ -8982,6 +8978,45 @@ module ts {
             }
         }
 
+        function checkElementTypeOfArrayOrString(arrayOrStringType: Type, expressionForError: Expression): Type {
+            Debug.assert(languageVersion < ScriptTarget.ES6);
+            var isJustString = allConstituentTypesHaveKind(arrayOrStringType, TypeFlags.StringLike);
+
+            // Check isJustString because removeTypesFromUnionType will only remove types if it doesn't result
+            // in an emptyObjectType. In this case, we actually do want the emptyObjectType.
+            var arrayType = isJustString ? emptyObjectType : removeTypesFromUnionType(arrayOrStringType, TypeFlags.StringLike, /*isTypeOfKind*/ true);
+            var hasStringConstituent = arrayOrStringType !== emptyObjectType && arrayOrStringType !== arrayType;
+
+            var reportedError = false;
+            if (hasStringConstituent && languageVersion < ScriptTarget.ES5) {
+                error(expressionForError, Diagnostics.Using_a_string_in_a_for_of_statement_is_only_supported_in_ECMAScript_5_and_higher);
+                reportedError = true;
+            }
+
+            if (isJustString) {
+                return stringType;
+            }
+
+            if (!isArrayLikeType(arrayType)) {
+                if (!reportedError) {
+                    error(expressionForError, Diagnostics.Type_0_is_not_an_array_type, typeToString(arrayType));
+                }
+                return hasStringConstituent ? stringType : unknownType;
+            }
+
+            var arrayElementType = getIndexTypeOfType(arrayType, IndexKind.Number) || unknownType;
+            if (hasStringConstituent) {
+                // This is just an optimization for the case where arrayOrStringType is string | string[]
+                if (arrayElementType.flags & TypeFlags.StringLike) {
+                    return stringType;
+                }
+
+                return getUnionType([arrayElementType, stringType]);
+            }
+
+            return arrayElementType;
+        }
+
         function checkBreakOrContinueStatement(node: BreakOrContinueStatement) {
             // Grammar checking
             checkGrammarStatementInAmbientContext(node) || checkGrammarBreakOrContinueStatement(node);

src/compiler/diagnosticInformationMap.generated.ts

@@ -338,6 +338,7 @@ module ts {
         The_left_hand_side_of_a_for_in_statement_cannot_be_a_destructuring_pattern: { code: 2491, category: DiagnosticCategory.Error, key: "The left-hand side of a 'for...in' statement cannot be a destructuring pattern." },
         Cannot_redeclare_identifier_0_in_catch_clause: { code: 2492, category: DiagnosticCategory.Error, key: "Cannot redeclare identifier '{0}' in catch clause" },
         Tuple_type_0_with_length_1_cannot_be_assigned_to_tuple_with_length_2: { code: 2493, category: DiagnosticCategory.Error, key: "Tuple type '{0}' with length '{1}' cannot be assigned to tuple with length '{2}'." },
+        Using_a_string_in_a_for_of_statement_is_only_supported_in_ECMAScript_5_and_higher: { code: 2494, category: DiagnosticCategory.Error, key: "Using a string in a 'for...of' statement is only supported in ECMAScript 5 and higher." },
         Import_declaration_0_is_using_private_name_1: { code: 4000, category: DiagnosticCategory.Error, key: "Import declaration '{0}' is using private name '{1}'." },
         Type_parameter_0_of_exported_class_has_or_is_using_private_name_1: { code: 4002, category: DiagnosticCategory.Error, key: "Type parameter '{0}' of exported class has or is using private name '{1}'." },
         Type_parameter_0_of_exported_interface_has_or_is_using_private_name_1: { code: 4004, category: DiagnosticCategory.Error, key: "Type parameter '{0}' of exported interface has or is using private name '{1}'." },

src/compiler/diagnosticMessages.json

@@ -1343,6 +1343,10 @@
         "category": "Error",
         "code": 2493
     },
+    "Using a string in a 'for...of' statement is only supported in ECMAScript 5 and higher.": {
+        "category": "Error",
+        "code": 2494
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",

tests/baselines/reference/ES5For-of10.types

@@ -0,0 +1,29 @@
+=== tests/cases/conformance/statements/for-ofStatements/ES5For-of10.ts ===
+function foo() {
+>foo : () => { x: number; }
+
+    return { x: 0 };
+>{ x: 0 } : { x: number; }
+>x : number
+}
+for (foo().x of []) {
+>foo().x : number
+>foo() : { x: number; }
+>foo : () => { x: number; }
+>x : number
+>[] : undefined[]
+
+    for (foo().x of [])
+>foo().x : number
+>foo() : { x: number; }
+>foo : () => { x: number; }
+>x : number
+>[] : undefined[]
+
+        var p = foo().x;
+>p : number
+>foo().x : number
+>foo() : { x: number; }
+>foo : () => { x: number; }
+>x : number
+}

tests/baselines/reference/ES5For-of11.types

@@ -0,0 +1,8 @@
+=== tests/cases/conformance/statements/for-ofStatements/ES5For-of11.ts ===
+var v;
+>v : any
+
+for (v of []) { }
+>v : any
+>[] : undefined[]
+

tests/baselines/reference/ES5For-of12.errors.txt

@@ -1,7 +1,7 @@
-tests/cases/conformance/statements/for-ofStatements/ES5For-of12.ts(1,1): error TS2482: 'for...of' statements are only available when targeting ECMAScript 6 or higher.
+tests/cases/conformance/statements/for-ofStatements/ES5For-of12.ts(1,6): error TS2461: Type 'undefined' is not an array type.
 
 
 ==== tests/cases/conformance/statements/for-ofStatements/ES5For-of12.ts (1 errors) ====
     for ([""] of []) { }
-    ~~~
-!!! error TS2482: 'for...of' statements are only available when targeting ECMAScript 6 or higher.
+         ~~~~
+!!! error TS2461: Type 'undefined' is not an array type.

tests/baselines/reference/ES5For-of13.types

@@ -0,0 +1,9 @@
+=== tests/cases/conformance/statements/for-ofStatements/ES5For-of13.ts ===
+for (let v of ['a', 'b', 'c']) {
+>v : string
+>['a', 'b', 'c'] : string[]
+
+    var x = v;
+>x : string
+>v : string
+}

tests/baselines/reference/ES5For-of24.types

@@ -0,0 +1,12 @@
+=== tests/cases/conformance/statements/for-ofStatements/ES5For-of24.ts ===
+var a = [1, 2, 3];
+>a : number[]
+>[1, 2, 3] : number[]
+
+for (var v of a) {
+>v : number
+>a : number[]
+
+    let a = 0;
+>a : number
+}

tests/baselines/reference/ES5For-of25.types

@@ -0,0 +1,15 @@
+=== tests/cases/conformance/statements/for-ofStatements/ES5For-of25.ts ===
+var a = [1, 2, 3];
+>a : number[]
+>[1, 2, 3] : number[]
+
+for (var v of a) {
+>v : number
+>a : number[]
+
+    v;
+>v : number
+
+    a;
+>a : number[]
+}

tests/baselines/reference/ES5For-of26.errors.txt

@@ -0,0 +1,10 @@
+tests/cases/conformance/statements/for-ofStatements/ES5For-of26.ts(1,10): error TS2461: Type 'number' is not an array type.
+
+
+==== tests/cases/conformance/statements/for-ofStatements/ES5For-of26.ts (1 errors) ====
+    for (var [a = 0, b = 1] of [2, 3]) {
+             ~~~~~~~~~~~~~~
+!!! error TS2461: Type 'number' is not an array type.
+        a;
+        b;
+    }

tests/baselines/reference/ES5For-of27.errors.txt

@@ -1,10 +1,13 @@
-tests/cases/conformance/statements/for-ofStatements/ES5For-of27.ts(1,1): error TS2482: 'for...of' statements are only available when targeting ECMAScript 6 or higher.
+tests/cases/conformance/statements/for-ofStatements/ES5For-of27.ts(1,11): error TS2459: Type 'number' has no property 'x' and no string index signature.
+tests/cases/conformance/statements/for-ofStatements/ES5For-of27.ts(1,21): error TS2459: Type 'number' has no property 'y' and no string index signature.
 
 
-==== tests/cases/conformance/statements/for-ofStatements/ES5For-of27.ts (1 errors) ====
+==== tests/cases/conformance/statements/for-ofStatements/ES5For-of27.ts (2 errors) ====
     for (var {x: a = 0, y: b = 1} of [2, 3]) {
-    ~~~
-!!! error TS2482: 'for...of' statements are only available when targeting ECMAScript 6 or higher.
+              ~
+!!! error TS2459: Type 'number' has no property 'x' and no string index signature.
+                        ~
+!!! error TS2459: Type 'number' has no property 'y' and no string index signature.
         a;
         b;
     }

tests/baselines/reference/ES5For-of28.errors.txt

@@ -0,0 +1,10 @@
+tests/cases/conformance/statements/for-ofStatements/ES5For-of28.ts(1,10): error TS2461: Type 'number' is not an array type.
+
+
+==== tests/cases/conformance/statements/for-ofStatements/ES5For-of28.ts (1 errors) ====
+    for (let [a = 0, b = 1] of [2, 3]) {
+             ~~~~~~~~~~~~~~
+!!! error TS2461: Type 'number' is not an array type.
+        a;
+        b;
+    }

tests/baselines/reference/ES5For-of29.errors.txt

@@ -1,10 +1,13 @@
-tests/cases/conformance/statements/for-ofStatements/ES5For-of29.ts(1,1): error TS2482: 'for...of' statements are only available when targeting ECMAScript 6 or higher.
+tests/cases/conformance/statements/for-ofStatements/ES5For-of29.ts(1,13): error TS2459: Type 'number' has no property 'x' and no string index signature.
+tests/cases/conformance/statements/for-ofStatements/ES5For-of29.ts(1,23): error TS2459: Type 'number' has no property 'y' and no string index signature.
 
 
-==== tests/cases/conformance/statements/for-ofStatements/ES5For-of29.ts (1 errors) ====
+==== tests/cases/conformance/statements/for-ofStatements/ES5For-of29.ts (2 errors) ====
     for (const {x: a = 0, y: b = 1} of [2, 3]) {
-    ~~~
-!!! error TS2482: 'for...of' statements are only available when targeting ECMAScript 6 or higher.
+                ~
+!!! error TS2459: Type 'number' has no property 'x' and no string index signature.
+                          ~
+!!! error TS2459: Type 'number' has no property 'y' and no string index signature.
         a;
         b;
     }

tests/baselines/reference/ES5For-of3.types

@@ -0,0 +1,9 @@
+=== tests/cases/conformance/statements/for-ofStatements/ES5For-of3.ts ===
+for (var v of ['a', 'b', 'c'])
+>v : string
+>['a', 'b', 'c'] : string[]
+
+    var x = v;
+>x : string
+>v : string
+

tests/baselines/reference/ES5For-of30.errors.txt

@@ -1,12 +1,12 @@
-tests/cases/conformance/statements/for-ofStatements/ES5For-of30.ts(3,1): error TS2482: 'for...of' statements are only available when targeting ECMAScript 6 or higher.
+tests/cases/conformance/statements/for-ofStatements/ES5For-of30.ts(3,6): error TS2461: Type 'string | number' is not an array type.
 
 
 ==== tests/cases/conformance/statements/for-ofStatements/ES5For-of30.ts (1 errors) ====
     var a: string, b: number;
     var tuple: [number, string] = [2, "3"];
     for ([a = 1, b = ""] of tuple) {
-    ~~~
-!!! error TS2482: 'for...of' statements are only available when targeting ECMAScript 6 or higher.
+         ~~~~~~~~~~~~~~~
+!!! error TS2461: Type 'string | number' is not an array type.
         a;
         b;
     }

tests/baselines/reference/ES5For-of31.errors.txt

@@ -1,12 +1,15 @@
-tests/cases/conformance/statements/for-ofStatements/ES5For-of31.ts(3,1): error TS2482: 'for...of' statements are only available when targeting ECMAScript 6 or higher.
+tests/cases/conformance/statements/for-ofStatements/ES5For-of31.ts(3,8): error TS2459: Type 'undefined' has no property 'a' and no string index signature.
+tests/cases/conformance/statements/for-ofStatements/ES5For-of31.ts(3,18): error TS2459: Type 'undefined' has no property 'b' and no string index signature.
 
 
-==== tests/cases/conformance/statements/for-ofStatements/ES5For-of31.ts (1 errors) ====
+==== tests/cases/conformance/statements/for-ofStatements/ES5For-of31.ts (2 errors) ====
     var a: string, b: number;
 
     for ({ a: b = 1, b: a = ""} of []) {
-    ~~~
-!!! error TS2482: 'for...of' statements are only available when targeting ECMAScript 6 or higher.
+           ~
+!!! error TS2459: Type 'undefined' has no property 'a' and no string index signature.
+                     ~
+!!! error TS2459: Type 'undefined' has no property 'b' and no string index signature.
         a;
         b;
     }

tests/baselines/reference/ES5For-of8.errors.txt

@@ -1,12 +1,12 @@
-tests/cases/conformance/statements/for-ofStatements/ES5For-of8.ts(4,1): error TS2482: 'for...of' statements are only available when targeting ECMAScript 6 or higher.
+tests/cases/conformance/statements/for-ofStatements/ES5For-of8.ts(4,6): error TS2322: Type 'string' is not assignable to type 'number'.
 
 
 ==== tests/cases/conformance/statements/for-ofStatements/ES5For-of8.ts (1 errors) ====
     function foo() {
         return { x: 0 };
     }
     for (foo().x of ['a', 'b', 'c']) {
-    ~~~
-!!! error TS2482: 'for...of' statements are only available when targeting ECMAScript 6 or higher.
+         ~~~~~~~
+!!! error TS2322: Type 'string' is not assignable to type 'number'.
         var p = foo().x;
     }

tests/baselines/reference/ES5For-of9.types

@@ -0,0 +1,30 @@
+=== tests/cases/conformance/statements/for-ofStatements/ES5For-of9.ts ===
+function foo() {
+>foo : () => { x: number; }
+
+    return { x: 0 };
+>{ x: 0 } : { x: number; }
+>x : number
+}
+for (foo().x of []) {
+>foo().x : number
+>foo() : { x: number; }
+>foo : () => { x: number; }
+>x : number
+>[] : undefined[]
+
+    for (foo().x of []) {
+>foo().x : number
+>foo() : { x: number; }
+>foo : () => { x: number; }
+>x : number
+>[] : undefined[]
+
+        var p = foo().x;
+>p : number
+>foo().x : number
+>foo() : { x: number; }
+>foo : () => { x: number; }
+>x : number
+    }
+}