Support destructuring in for-in

src/compiler/checker.ts

@@ -41529,10 +41529,6 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         //   VarDecl must be a variable declaration without a type annotation that declares a variable of type Any,
         //   and Expr must be an expression of type Any, an object type, or a type parameter type.
         if (node.initializer.kind === SyntaxKind.VariableDeclarationList) {
-            const variable = (node.initializer as VariableDeclarationList).declarations[0];
-            if (variable && isBindingPattern(variable.name)) {
-                error(variable.name, Diagnostics.The_left_hand_side_of_a_for_in_statement_cannot_be_a_destructuring_pattern);
-            }
             checkVariableDeclarationList(node.initializer as VariableDeclarationList);
         }
         else {
@@ -41542,8 +41538,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             //   and Expr must be an expression of type Any, an object type, or a type parameter type.
             const varExpr = node.initializer;
             const leftType = checkExpression(varExpr);
-            if (varExpr.kind === SyntaxKind.ArrayLiteralExpression || varExpr.kind === SyntaxKind.ObjectLiteralExpression) {
-                error(varExpr, Diagnostics.The_left_hand_side_of_a_for_in_statement_cannot_be_a_destructuring_pattern);
+            if (isAssignmentPattern(varExpr)) {
+                checkDestructuringAssignment(varExpr, getIndexTypeOrString(rightType));
             }
             else if (!isTypeAssignableTo(getIndexTypeOrString(rightType), leftType)) {
                 error(varExpr, Diagnostics.The_left_hand_side_of_a_for_in_statement_must_be_of_type_string_or_any);

src/compiler/diagnosticMessages.json

@@ -2372,10 +2372,6 @@
         "category": "Error",
         "code": 2490
     },
-    "The left-hand side of a 'for...in' statement cannot be a destructuring pattern.": {
-        "category": "Error",
-        "code": 2491
-    },
     "Cannot redeclare identifier '{0}' in catch clause.": {
         "category": "Error",
         "code": 2492

src/compiler/transformers/es2015.ts

@@ -51,6 +51,7 @@ import {
     flattenDestructuringAssignment,
     flattenDestructuringBinding,
     FlattenLevel,
+    ForInOrOfStatement,
     ForInStatement,
     ForOfStatement,
     ForStatement,
@@ -85,6 +86,7 @@ import {
     isArrayLiteralExpression,
     isArrowFunction,
     isAssignmentExpression,
+    isAssignmentPattern,
     isBinaryExpression,
     isBindingPattern,
     isBlock,
@@ -2684,11 +2686,16 @@ export function transformES2015(context: TransformationContext): (x: SourceFile
     }
 
     function visitForInStatement(node: ForInStatement, outermostLabeledStatement: LabeledStatement | undefined) {
+        const needsConversionForDestructuring = node.initializer.kind === SyntaxKind.VariableDeclarationList ?
+            isBindingPattern((node.initializer as VariableDeclarationList).declarations[0].name) :
+            isAssignmentPattern(node.initializer);
         return visitIterationStatementWithFacts(
             HierarchyFacts.ForInOrForOfStatementExcludes,
             HierarchyFacts.ForInOrForOfStatementIncludes,
             node,
-            outermostLabeledStatement);
+            outermostLabeledStatement,
+            needsConversionForDestructuring ? convertForInStatementForDestructuring : undefined
+        );
     }
 
     function visitForOfStatement(node: ForOfStatement, outermostLabeledStatement: LabeledStatement | undefined): VisitResult<Statement> {
@@ -2700,7 +2707,7 @@ export function transformES2015(context: TransformationContext): (x: SourceFile
             compilerOptions.downlevelIteration ? convertForOfStatementForIterable : convertForOfStatementForArray);
     }
 
-    function convertForOfStatementHead(node: ForOfStatement, boundValue: Expression, convertedLoopBodyStatements: Statement[] | undefined) {
+    function convertForInOrOfStatementHead(node: ForInOrOfStatement, boundValue: Expression, convertedLoopBodyStatements: Statement[] | undefined) {
         const statements: Statement[] = [];
         const initializer = node.initializer;
         if (isVariableDeclarationList(initializer)) {
@@ -2800,6 +2807,36 @@ export function transformES2015(context: TransformationContext): (x: SourceFile
         );
     }
 
+    function convertForInStatementForDestructuring(node: ForInStatement, outermostLabeledStatement: LabeledStatement | undefined, convertedLoopBodyStatements: Statement[] | undefined): Statement {
+        const lhsReference = factory.createUniqueName("key");
+
+        const forInStatement = setEmitFlags(
+            setTextRange(
+                factory.createForInStatement(
+                    /*initializer*/ setEmitFlags(
+                        setTextRange(
+                            factory.createVariableDeclarationList([
+                                setTextRange(factory.createVariableDeclaration(lhsReference, /*exclamationToken*/ undefined, /*type*/ undefined, /*initializer*/ undefined), node.initializer),
+                            ]),
+                            node.expression
+                        ),
+                        EmitFlags.NoHoisting
+                    ),
+                    /*expression*/ node.expression,
+                    /*statement*/ convertForInOrOfStatementHead(
+                        node,
+                        lhsReference,
+                        convertedLoopBodyStatements
+                    )
+                ),
+                /*location*/ node
+            ),
+            EmitFlags.NoTokenTrailingSourceMaps
+        );
+
+        return factory.restoreEnclosingLabel(forInStatement, outermostLabeledStatement, convertedLoopState && resetLabel);
+    }
+
     function convertForOfStatementForArray(node: ForOfStatement, outermostLabeledStatement: LabeledStatement | undefined, convertedLoopBodyStatements: Statement[] | undefined): Statement {
         // The following ES6 code:
         //
@@ -2856,7 +2893,7 @@ export function transformES2015(context: TransformationContext): (x: SourceFile
                     node.expression
                 ),
                 /*incrementor*/ setTextRange(factory.createPostfixIncrement(counter), node.expression),
-                /*statement*/ convertForOfStatementHead(
+                /*statement*/ convertForInOrOfStatementHead(
                     node,
                     factory.createElementAccessExpression(rhsReference, counter),
                     convertedLoopBodyStatements
@@ -2867,7 +2904,6 @@ export function transformES2015(context: TransformationContext): (x: SourceFile
 
         // Disable trailing source maps for the OpenParenToken to align source map emit with the old emitter.
         setEmitFlags(forStatement, EmitFlags.NoTokenTrailingSourceMaps);
-        setTextRange(forStatement, node);
         return factory.restoreEnclosingLabel(forStatement, outermostLabeledStatement, convertedLoopState && resetLabel);
     }
 
@@ -2905,7 +2941,7 @@ export function transformES2015(context: TransformationContext): (x: SourceFile
                     ),
                     /*condition*/ factory.createLogicalNot(factory.createPropertyAccessExpression(result, "done")),
                     /*incrementor*/ factory.createAssignment(result, next),
-                    /*statement*/ convertForOfStatementHead(
+                    /*statement*/ convertForInOrOfStatementHead(
                         node,
                         factory.createPropertyAccessExpression(result, "value"),
                         convertedLoopBodyStatements

src/testRunner/unittests/evaluation/destructuring.ts

@@ -65,4 +65,36 @@ describe("unittests:: evaluation:: destructuring", () => {
             assert.deepEqual(result.output, { a: 1, b: { y: 2 } });
         });
     });
+    describe("correct evaluation in for-in with array pattern", () => {
+        it("ES5", () => {
+            const result = evaluator.evaluateTypeScript(`
+                export let output: string;
+                for (const [first] in { test: 1 }) { output = first; }
+            `, { target: ts.ScriptTarget.ES5, downlevelIteration: true });
+            assert.deepEqual(result.output, "t");
+        });
+        it("ESNext", () => {
+            const result = evaluator.evaluateTypeScript(`
+                export let output: string;
+                for (const [first] in { test: 1 }) { output = first; }
+            `, { target: ts.ScriptTarget.ESNext });
+            assert.deepEqual(result.output, "t");
+        });
+    });
+    describe("correct evaluation in for-in with object pattern", () => {
+        it("ES5", () => {
+            const result = evaluator.evaluateTypeScript(`
+                export let output: number;
+                for (const { length } in { test: 1 }) { output = length; }
+            `, { target: ts.ScriptTarget.ES5, downlevelIteration: true });
+            assert.deepEqual(result.output, 4);
+        });
+        it("ESNext", () => {
+            const result = evaluator.evaluateTypeScript(`
+                export let output: number;
+                for (const { length } in { test: 1 }) { output = length; }
+            `, { target: ts.ScriptTarget.ESNext });
+            assert.deepEqual(result.output, 4);
+        });
+    });
 });

tests/baselines/reference/for-inStatementsDestructuring.errors.txt

@@ -1,10 +1,7 @@
 for-inStatementsDestructuring.ts(1,10): error TS2461: Type 'string' is not an array type.
-for-inStatementsDestructuring.ts(1,10): error TS2491: The left-hand side of a 'for...in' statement cannot be a destructuring pattern.
 
 
-==== for-inStatementsDestructuring.ts (2 errors) ====
+==== for-inStatementsDestructuring.ts (1 errors) ====
     for (var [a, b] in []) {}
              ~~~~~~
-!!! error TS2461: Type 'string' is not an array type.
-             ~~~~~~
-!!! error TS2491: The left-hand side of a 'for...in' statement cannot be a destructuring pattern.
+!!! error TS2461: Type 'string' is not an array type.

tests/baselines/reference/for-inStatementsDestructuring.js

@@ -4,4 +4,6 @@
 for (var [a, b] in []) {}
 
 //// [for-inStatementsDestructuring.js]
-for (var _a = void 0, a = _a[0], b = _a[1] in []) { }
+for (var key_1 in []) {
+    var a = key_1[0], b = key_1[1];
+}

tests/baselines/reference/for-inStatementsDestructuring2.errors.txt

@@ -1,12 +1,9 @@
-for-inStatementsDestructuring2.ts(1,10): error TS2491: The left-hand side of a 'for...in' statement cannot be a destructuring pattern.
 for-inStatementsDestructuring2.ts(1,11): error TS2339: Property 'a' does not exist on type 'String'.
 for-inStatementsDestructuring2.ts(1,14): error TS2339: Property 'b' does not exist on type 'String'.
 
 
-==== for-inStatementsDestructuring2.ts (3 errors) ====
+==== for-inStatementsDestructuring2.ts (2 errors) ====
     for (var {a, b} in []) {}
-             ~~~~~~
-!!! error TS2491: The left-hand side of a 'for...in' statement cannot be a destructuring pattern.
               ~
 !!! error TS2339: Property 'a' does not exist on type 'String'.
                  ~

tests/baselines/reference/for-inStatementsDestructuring2.js

@@ -4,4 +4,6 @@
 for (var {a, b} in []) {}
 
 //// [for-inStatementsDestructuring2.js]
-for (var _a = void 0, a = _a.a, b = _a.b in []) { }
+for (var key_1 in []) {
+    var a = key_1.a, b = key_1.b;
+}

tests/baselines/reference/for-inStatementsDestructuring3.errors.txt

@@ -1,8 +1,8 @@
-for-inStatementsDestructuring3.ts(2,6): error TS2491: The left-hand side of a 'for...in' statement cannot be a destructuring pattern.
+for-inStatementsDestructuring3.ts(2,6): error TS2461: Type '"length" | "toString" | "toLocaleString" | "pop" | "push" | "concat" | "join" | "reverse" | "shift" | "slice" | "sort" | "splice" | "unshift" | "indexOf" | "lastIndexOf" | "every" | "some" | "forEach" | "map" | "filter" | "reduce" | "reduceRight"' is not an array type.
 
 
 ==== for-inStatementsDestructuring3.ts (1 errors) ====
     var a, b;
     for ([a, b] in []) { }
          ~~~~~~
-!!! error TS2491: The left-hand side of a 'for...in' statement cannot be a destructuring pattern.
+!!! error TS2461: Type '"length" | "toString" | "toLocaleString" | "pop" | "push" | "concat" | "join" | "reverse" | "shift" | "slice" | "sort" | "splice" | "unshift" | "indexOf" | "lastIndexOf" | "every" | "some" | "forEach" | "map" | "filter" | "reduce" | "reduceRight"' is not an array type.

tests/baselines/reference/for-inStatementsDestructuring4.errors.txt

@@ -1,8 +1,11 @@
-for-inStatementsDestructuring4.ts(2,6): error TS2491: The left-hand side of a 'for...in' statement cannot be a destructuring pattern.
+for-inStatementsDestructuring4.ts(2,7): error TS2339: Property 'a' does not exist on type '"length" | "toString" | "toLocaleString" | "pop" | "push" | "concat" | "join" | "reverse" | "shift" | "slice" | "sort" | "splice" | "unshift" | "indexOf" | "lastIndexOf" | "every" | "some" | "forEach" | "map" | "filter" | "reduce" | "reduceRight"'.
+for-inStatementsDestructuring4.ts(2,10): error TS2339: Property 'b' does not exist on type '"length" | "toString" | "toLocaleString" | "pop" | "push" | "concat" | "join" | "reverse" | "shift" | "slice" | "sort" | "splice" | "unshift" | "indexOf" | "lastIndexOf" | "every" | "some" | "forEach" | "map" | "filter" | "reduce" | "reduceRight"'.
 
 
-==== for-inStatementsDestructuring4.ts (1 errors) ====
+==== for-inStatementsDestructuring4.ts (2 errors) ====
     var a, b;
     for ({a, b} in []) { }
-         ~~~~~~
-!!! error TS2491: The left-hand side of a 'for...in' statement cannot be a destructuring pattern.
+          ~
+!!! error TS2339: Property 'a' does not exist on type '"length" | "toString" | "toLocaleString" | "pop" | "push" | "concat" | "join" | "reverse" | "shift" | "slice" | "sort" | "splice" | "unshift" | "indexOf" | "lastIndexOf" | "every" | "some" | "forEach" | "map" | "filter" | "reduce" | "reduceRight"'.
+             ~
+!!! error TS2339: Property 'b' does not exist on type '"length" | "toString" | "toLocaleString" | "pop" | "push" | "concat" | "join" | "reverse" | "shift" | "slice" | "sort" | "splice" | "unshift" | "indexOf" | "lastIndexOf" | "every" | "some" | "forEach" | "map" | "filter" | "reduce" | "reduceRight"'.

tests/baselines/reference/for-inStatementsDestructuring4.js

@@ -6,4 +6,6 @@ for ({a, b} in []) { }
 
 //// [for-inStatementsDestructuring4.js]
 var a, b;
-for ({ a: a, b: b } in []) { }
+for (var key_1 in []) {
+    a = key_1.a, b = key_1.b;
+}

tests/baselines/reference/for-inStatementsDestructuring5(target=es5).errors.txt

@@ -0,0 +1,11 @@
+for-inStatementsDestructuring5.ts(4,6): error TS2461: Type 'string' is not an array type.
+
+
+==== for-inStatementsDestructuring5.ts (1 errors) ====
+    let a: string;
+    declare let obj: { [s: string]: string };
+
+    for ([a] in obj) {}
+         ~~~
+!!! error TS2461: Type 'string' is not an array type.
+

tests/baselines/reference/for-inStatementsDestructuring5(target=es5).js

@@ -0,0 +1,12 @@
+//// [tests/cases/conformance/statements/for-inStatements/for-inStatementsDestructuring5.ts] ////
+
+//// [for-inStatementsDestructuring5.ts]
+let a: string;
+declare let obj: { [s: string]: string };
+
+for ([a] in obj) {}
+
+
+//// [for-inStatementsDestructuring5.js]
+var a;
+for ([a] in obj) { }

tests/baselines/reference/for-inStatementsDestructuring5(target=es5).symbols

@@ -0,0 +1,14 @@
+//// [tests/cases/conformance/statements/for-inStatements/for-inStatementsDestructuring5.ts] ////
+
+=== for-inStatementsDestructuring5.ts ===
+let a: string;
+>a : Symbol(a, Decl(for-inStatementsDestructuring5.ts, 0, 3))
+
+declare let obj: { [s: string]: string };
+>obj : Symbol(obj, Decl(for-inStatementsDestructuring5.ts, 1, 11))
+>s : Symbol(s, Decl(for-inStatementsDestructuring5.ts, 1, 20))
+
+for ([a] in obj) {}
+>a : Symbol(a, Decl(for-inStatementsDestructuring5.ts, 0, 3))
+>obj : Symbol(obj, Decl(for-inStatementsDestructuring5.ts, 1, 11))
+

tests/baselines/reference/for-inStatementsDestructuring5(target=es5).types

@@ -0,0 +1,15 @@
+//// [tests/cases/conformance/statements/for-inStatements/for-inStatementsDestructuring5.ts] ////
+
+=== for-inStatementsDestructuring5.ts ===
+let a: string;
+>a : string
+
+declare let obj: { [s: string]: string };
+>obj : { [s: string]: string; }
+>s : string
+
+for ([a] in obj) {}
+>[a] : [string]
+>a : string
+>obj : { [s: string]: string; }
+

tests/baselines/reference/for-inStatementsDestructuring5(target=esnext).js

@@ -0,0 +1,12 @@
+//// [tests/cases/conformance/statements/for-inStatements/for-inStatementsDestructuring5.ts] ////
+
+//// [for-inStatementsDestructuring5.ts]
+let a: string;
+declare let obj: { [s: string]: string };
+
+for ([a] in obj) {}
+
+
+//// [for-inStatementsDestructuring5.js]
+let a;
+for ([a] in obj) { }

tests/baselines/reference/for-inStatementsDestructuring5(target=esnext).symbols

@@ -0,0 +1,14 @@
+//// [tests/cases/conformance/statements/for-inStatements/for-inStatementsDestructuring5.ts] ////
+
+=== for-inStatementsDestructuring5.ts ===
+let a: string;
+>a : Symbol(a, Decl(for-inStatementsDestructuring5.ts, 0, 3))
+
+declare let obj: { [s: string]: string };
+>obj : Symbol(obj, Decl(for-inStatementsDestructuring5.ts, 1, 11))
+>s : Symbol(s, Decl(for-inStatementsDestructuring5.ts, 1, 20))
+
+for ([a] in obj) {}
+>a : Symbol(a, Decl(for-inStatementsDestructuring5.ts, 0, 3))
+>obj : Symbol(obj, Decl(for-inStatementsDestructuring5.ts, 1, 11))
+

tests/baselines/reference/for-inStatementsDestructuring5(target=esnext).types

@@ -0,0 +1,15 @@
+//// [tests/cases/conformance/statements/for-inStatements/for-inStatementsDestructuring5.ts] ////
+
+=== for-inStatementsDestructuring5.ts ===
+let a: string;
+>a : string
+
+declare let obj: { [s: string]: string };
+>obj : { [s: string]: string; }
+>s : string
+
+for ([a] in obj) {}
+>[a] : [string]
+>a : string
+>obj : { [s: string]: string; }
+

tests/baselines/reference/for-inStatementsDestructuring6(target=es5).errors.txt

@@ -0,0 +1,12 @@
+for-inStatementsDestructuring6.ts(5,6): error TS2461: Type 'string' is not an array type.
+
+
+==== for-inStatementsDestructuring6.ts (1 errors) ====
+    let b: number;
+    let c: string[];
+    declare let obj: { [s: string]: string };
+
+    for ([b, ...c] in obj) {}
+         ~~~~~~~~~
+!!! error TS2461: Type 'string' is not an array type.
+

tests/baselines/reference/for-inStatementsDestructuring6(target=es5).js

@@ -0,0 +1,16 @@
+//// [tests/cases/conformance/statements/for-inStatements/for-inStatementsDestructuring6.ts] ////
+
+//// [for-inStatementsDestructuring6.ts]
+let b: number;
+let c: string[];
+declare let obj: { [s: string]: string };
+
+for ([b, ...c] in obj) {}
+
+
+//// [for-inStatementsDestructuring6.js]
+var b;
+var c;
+for (var key_1 in obj) {
+    b = key_1[0], c = key_1.slice(1);
+}