Support assignment patterns in 'for...of' statements

src/compiler/checker.ts

@@ -7411,7 +7411,7 @@ module ts {
                     return rightType;
             }
 
-            // Return type is true if there was no error, false if there was an error.
+            // Return true if there was no error, false if there was an error.
             function checkForDisallowedESSymbolOperand(operator: SyntaxKind): boolean {
                 var offendingSymbolOperand =
                     someConstituentTypeHasKind(leftType, TypeFlags.ESSymbol) ? node.left :
@@ -8758,7 +8758,7 @@ module ts {
                 grammarErrorOnFirstToken(node, Diagnostics.for_of_statements_are_only_available_when_targeting_ECMAScript_6_or_higher);
                 return;
             }
-
+            
             checkGrammarForInOrForOfStatement(node)
 
             // Check the LHS and RHS
@@ -8769,17 +8769,27 @@ module ts {
             }
             else {
                 var varExpr = <Expression>node.initializer;
-                var leftType = checkExpression(varExpr);
-                checkReferenceExpression(varExpr, Diagnostics.Invalid_left_hand_side_in_for_of_statement, Diagnostics.The_left_hand_side_of_a_for_of_statement_cannot_be_a_previously_defined_constant);
                 var rightType = checkExpression(node.expression);
                 var iteratedType = getIteratedType(rightType, node.expression);
                 
-                // iteratedType will be undefined if the rightType was missing properties/signatures
-                // required to get it's 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.
-                if (iteratedType) {
-                    checkTypeAssignableTo(iteratedType, leftType, varExpr, /*headMessage*/ undefined);
+                // There may be a destructuring assignment on the left side
+                if (varExpr.kind === SyntaxKind.ArrayLiteralExpression || varExpr.kind === SyntaxKind.ObjectLiteralExpression) {
+                    // iteratedType may be undefined. In this case, we still want to check the structure of
+                    // varExpr, in particular making sure it's a valid LeftHandSideExpression. But we'd like
+                    // to short circuit the type relation checking as much as possible, so we pass the unknownType.
+                    checkDestructuringAssignment(varExpr, iteratedType || unknownType);
+                }
+                else {
+                    var leftType = checkExpression(varExpr);
+                    checkReferenceExpression(varExpr, Diagnostics.Invalid_left_hand_side_in_for_of_statement, Diagnostics.The_left_hand_side_of_a_for_of_statement_cannot_be_a_previously_defined_constant);
+                
+                    // iteratedType will be undefined if the rightType was missing properties/signatures
+                    // required to get it's 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.
+                    if (iteratedType) {
+                        checkTypeAssignableTo(iteratedType, leftType, varExpr, /*headMessage*/ undefined);
+                    }
                 }
             }
 

tests/baselines/reference/for-of45.js

@@ -0,0 +1,15 @@
+//// [for-of45.ts]
+var k: string, v: boolean;
+var map = new Map([["", true]]);
+for ([k = "", v = false] of map) {
+    k;
+    v;
+}
+
+//// [for-of45.js]
+var k, v;
+var map = new Map([["", true]]);
+for ([k = "", v = false] of map) {
+    k;
+    v;
+}

tests/baselines/reference/for-of45.types

@@ -0,0 +1,26 @@
+=== tests/cases/conformance/es6/for-ofStatements/for-of45.ts ===
+var k: string, v: boolean;
+>k : string
+>v : boolean
+
+var map = new Map([["", true]]);
+>map : Map<string, boolean>
+>new Map([["", true]]) : Map<string, boolean>
+>Map : MapConstructor
+>[["", true]] : [string, boolean][]
+>["", true] : [string, boolean]
+
+for ([k = "", v = false] of map) {
+>[k = "", v = false] : (string | boolean)[]
+>k = "" : string
+>k : string
+>v = false : boolean
+>v : boolean
+>map : Map<string, boolean>
+
+    k;
+>k : string
+
+    v;
+>v : boolean
+}

tests/baselines/reference/for-of46.errors.txt

@@ -0,0 +1,15 @@
+tests/cases/conformance/es6/for-ofStatements/for-of46.ts(3,7): error TS2322: Type 'boolean' is not assignable to type 'string'.
+tests/cases/conformance/es6/for-ofStatements/for-of46.ts(3,18): error TS2322: Type 'string' is not assignable to type 'boolean'.
+
+
+==== tests/cases/conformance/es6/for-ofStatements/for-of46.ts (2 errors) ====
+    var k: string, v: boolean;
+    var map = new Map([["", true]]);
+    for ([k = false, v = ""] of map) {
+          ~
+!!! error TS2322: Type 'boolean' is not assignable to type 'string'.
+                     ~
+!!! error TS2322: Type 'string' is not assignable to type 'boolean'.
+        k;
+        v;
+    }

tests/baselines/reference/for-of46.js

@@ -0,0 +1,15 @@
+//// [for-of46.ts]
+var k: string, v: boolean;
+var map = new Map([["", true]]);
+for ([k = false, v = ""] of map) {
+    k;
+    v;
+}
+
+//// [for-of46.js]
+var k, v;
+var map = new Map([["", true]]);
+for ([k = false, v = ""] of map) {
+    k;
+    v;
+}

tests/baselines/reference/for-of47.errors.txt

@@ -0,0 +1,13 @@
+tests/cases/conformance/es6/for-ofStatements/for-of47.ts(4,13): error TS2322: Type 'boolean' is not assignable to type 'number'.
+
+
+==== tests/cases/conformance/es6/for-ofStatements/for-of47.ts (1 errors) ====
+    var x: string, y: number;
+    var array = [{ x: "", y: true }]
+    enum E { x }
+    for ({x, y: y = E.x} of array) {
+                ~
+!!! error TS2322: Type 'boolean' is not assignable to type 'number'.
+        x;
+        y;
+    }

tests/baselines/reference/for-of47.js

@@ -0,0 +1,20 @@
+//// [for-of47.ts]
+var x: string, y: number;
+var array = [{ x: "", y: true }]
+enum E { x }
+for ({x, y: y = E.x} of array) {
+    x;
+    y;
+}
+
+//// [for-of47.js]
+var x, y;
+var array = [{ x: "", y: true }];
+var E;
+(function (E) {
+    E[E["x"] = 0] = "x";
+})(E || (E = {}));
+for ({ x, y: y = 0 /* x */ } of array) {
+    x;
+    y;
+}

tests/cases/conformance/es6/for-ofStatements/for-of45.ts

@@ -0,0 +1,7 @@
+//@target: ES6
+var k: string, v: boolean;
+var map = new Map([["", true]]);
+for ([k = "", v = false] of map) {
+    k;
+    v;
+}

tests/cases/conformance/es6/for-ofStatements/for-of46.ts

@@ -0,0 +1,7 @@
+//@target: ES6
+var k: string, v: boolean;
+var map = new Map([["", true]]);
+for ([k = false, v = ""] of map) {
+    k;
+    v;
+}

tests/cases/conformance/es6/for-ofStatements/for-of47.ts

@@ -0,0 +1,8 @@
+//@target: ES6
+var x: string, y: number;
+var array = [{ x: "", y: true }]
+enum E { x }
+for ({x, y: y = E.x} of array) {
+    x;
+    y;
+}