use string as type of variable in ForInStatement

src/compiler/checker.ts

@@ -2510,9 +2510,9 @@ namespace ts {
 
         // Return the inferred type for a variable, parameter, or property declaration
         function getTypeForVariableLikeDeclaration(declaration: VariableLikeDeclaration): Type {
-            // A variable declared in a for..in statement is always of type any
+            // A variable declared in a for..in statement is always of type string
             if (declaration.parent.parent.kind === SyntaxKind.ForInStatement) {
-                return anyType;
+                return stringType;
             }
 
             if (declaration.parent.parent.kind === SyntaxKind.ForOfStatement) {
@@ -12691,6 +12691,7 @@ namespace ts {
                 }
             }
 
+            const isVariableInForInStatement = node.parent.parent.kind === SyntaxKind.ForInStatement;
             // For a binding pattern, check contained binding elements
             if (isBindingPattern(node.name)) {
                 forEach((<BindingPattern>node.name).elements, checkSourceElement);
@@ -12702,7 +12703,9 @@ namespace ts {
             }
             // For a binding pattern, validate the initializer and exit
             if (isBindingPattern(node.name)) {
-                if (node.initializer) {
+                // do not check initializer if this is variable in ForIn statement.
+                // this situation is already illegal so no need to report misleading errors.
+                if (node.initializer && !isVariableInForInStatement) {
                     checkTypeAssignableTo(checkExpressionCached(node.initializer), getWidenedTypeForVariableLikeDeclaration(node), node, /*headMessage*/ undefined);
                     checkParameterInitializer(node);
                 }
@@ -12712,7 +12715,9 @@ namespace ts {
             const type = getTypeOfVariableOrParameterOrProperty(symbol);
             if (node === symbol.valueDeclaration) {
                 // Node is the primary declaration of the symbol, just validate the initializer
-                if (node.initializer) {
+                // NOTE: do not check initializer if this is variable in ForIn statement.
+                // this situation is already illegal so no need to report misleading errors.
+                if (node.initializer && !isVariableInForInStatement) {
                     checkTypeAssignableTo(checkExpressionCached(node.initializer), type, node, /*headMessage*/ undefined);
                     checkParameterInitializer(node);
                 }

tests/baselines/reference/capturedLetConstInLoop1.types

@@ -1,18 +1,18 @@
 === tests/cases/compiler/capturedLetConstInLoop1.ts ===
 //==== let
 for (let x in {}) {
->x : any
+>x : string
 >{} : {}
 
     (function() { return x});
->(function() { return x}) : () => any
->function() { return x} : () => any
->x : any
+>(function() { return x}) : () => string
+>function() { return x} : () => string
+>x : string
 
     (() => x);
->(() => x) : () => any
->() => x : () => any
->x : any
+>(() => x) : () => string
+>() => x : () => string
+>x : string
 }
 
 for (let x of []) {
@@ -216,18 +216,18 @@ for (let y = 0; y < 1; ++y) {
 
 //=========const
 for (const x in {}) {
->x : any
+>x : string
 >{} : {}
 
     (function() { return x});
->(function() { return x}) : () => any
->function() { return x} : () => any
->x : any
+>(function() { return x}) : () => string
+>function() { return x} : () => string
+>x : string
 
     (() => x);
->(() => x) : () => any
->() => x : () => any
->x : any
+>(() => x) : () => string
+>() => x : () => string
+>x : string
 }
 
 for (const x of []) {

tests/baselines/reference/capturedLetConstInLoop1_ES6.types

@@ -1,18 +1,18 @@
 === tests/cases/compiler/capturedLetConstInLoop1_ES6.ts ===
 //==== let
 for (let x in {}) {
->x : any
+>x : string
 >{} : {}
 
     (function() { return x});
->(function() { return x}) : () => any
->function() { return x} : () => any
->x : any
+>(function() { return x}) : () => string
+>function() { return x} : () => string
+>x : string
 
     (() => x);
->(() => x) : () => any
->() => x : () => any
->x : any
+>(() => x) : () => string
+>() => x : () => string
+>x : string
 }
 
 for (let x of []) {
@@ -216,18 +216,18 @@ for (let y = 0; y < 1; ++y) {
 
 //=========const
 for (const x in {}) {
->x : any
+>x : string
 >{} : {}
 
     (function() { return x});
->(function() { return x}) : () => any
->function() { return x} : () => any
->x : any
+>(function() { return x}) : () => string
+>function() { return x} : () => string
+>x : string
 
     (() => x);
->(() => x) : () => any
->() => x : () => any
->x : any
+>(() => x) : () => string
+>() => x : () => string
+>x : string
 }
 
 for (const x of []) {

tests/baselines/reference/capturedLetConstInLoop2.types

@@ -37,7 +37,7 @@ function foo0_1(x) {
 >x : any
 
     for (let x in []) {
->x : any
+>x : string
 >[] : undefined[]
 
         let a = arguments.length;
@@ -47,17 +47,17 @@ function foo0_1(x) {
 >length : number
 
         (function() { return x + a });
->(function() { return x + a }) : () => any
->function() { return x + a } : () => any
->x + a : any
->x : any
+>(function() { return x + a }) : () => string
+>function() { return x + a } : () => string
+>x + a : string
+>x : string
 >a : number
 
         (() => x + a);
->(() => x + a) : () => any
->() => x + a : () => any
->x + a : any
->x : any
+>(() => x + a) : () => string
+>() => x + a : () => string
+>x + a : string
+>x : string
 >a : number
     }
 }
@@ -400,7 +400,7 @@ function foo0_1_c(x) {
 >x : any
 
     for (const x in []) {
->x : any
+>x : string
 >[] : undefined[]
 
         const a = arguments.length;
@@ -410,17 +410,17 @@ function foo0_1_c(x) {
 >length : number
 
         (function() { return x + a });
->(function() { return x + a }) : () => any
->function() { return x + a } : () => any
->x + a : any
->x : any
+>(function() { return x + a }) : () => string
+>function() { return x + a } : () => string
+>x + a : string
+>x : string
 >a : number
 
         (() => x + a);
->(() => x + a) : () => any
->() => x + a : () => any
->x + a : any
->x : any
+>(() => x + a) : () => string
+>() => x + a : () => string
+>x + a : string
+>x : string
 >a : number
     }
 }

tests/baselines/reference/capturedLetConstInLoop2_ES6.types

@@ -36,7 +36,7 @@ function foo0_1(x) {
 >x : any
 
     for (let x in []) {
->x : any
+>x : string
 >[] : undefined[]
 
         let a = arguments.length;
@@ -46,17 +46,17 @@ function foo0_1(x) {
 >length : number
 
         (function() { return x + a });
->(function() { return x + a }) : () => any
->function() { return x + a } : () => any
->x + a : any
->x : any
+>(function() { return x + a }) : () => string
+>function() { return x + a } : () => string
+>x + a : string
+>x : string
 >a : number
 
         (() => x + a);
->(() => x + a) : () => any
->() => x + a : () => any
->x + a : any
->x : any
+>(() => x + a) : () => string
+>() => x + a : () => string
+>x + a : string
+>x : string
 >a : number
     }
 }
@@ -399,7 +399,7 @@ function foo0_1_c(x) {
 >x : any
 
     for (const x in []) {
->x : any
+>x : string
 >[] : undefined[]
 
         const a = arguments.length;
@@ -409,17 +409,17 @@ function foo0_1_c(x) {
 >length : number
 
         (function() { return x + a });
->(function() { return x + a }) : () => any
->function() { return x + a } : () => any
->x + a : any
->x : any
+>(function() { return x + a }) : () => string
+>function() { return x + a } : () => string
+>x + a : string
+>x : string
 >a : number
 
         (() => x + a);
->(() => x + a) : () => any
->() => x + a : () => any
->x + a : any
->x : any
+>(() => x + a) : () => string
+>() => x + a : () => string
+>x + a : string
+>x : string
 >a : number
     }
 }

tests/baselines/reference/capturedLetConstInLoop3.types

@@ -42,32 +42,32 @@ function foo0_1(x) {
 >x : any
 
     for (let x in []) {
->x : any
+>x : string
 >[] : undefined[]
 
         var v = x;
->v : any
->x : any
+>v : string
+>x : string
 
         (function() { return x + v });
->(function() { return x + v }) : () => any
->function() { return x + v } : () => any
->x + v : any
->x : any
->v : any
+>(function() { return x + v }) : () => string
+>function() { return x + v } : () => string
+>x + v : string
+>x : string
+>v : string
 
         (() => x + v);
->(() => x + v) : () => any
->() => x + v : () => any
->x + v : any
->x : any
->v : any
+>(() => x + v) : () => string
+>() => x + v : () => string
+>x + v : string
+>x : string
+>v : string
     }
 
     use(v);
 >use(v) : any
 >use : (a: any) => any
->v : any
+>v : string
 }
 
 function foo1(x) {
@@ -438,32 +438,32 @@ function foo0_1_c(x) {
 >x : any
 
     for (const x in []) {
->x : any
+>x : string
 >[] : undefined[]
 
         var v = x;
->v : any
->x : any
+>v : string
+>x : string
 
         (function() { return x + v });
->(function() { return x + v }) : () => any
->function() { return x + v } : () => any
->x + v : any
->x : any
->v : any
+>(function() { return x + v }) : () => string
+>function() { return x + v } : () => string
+>x + v : string
+>x : string
+>v : string
 
         (() => x + v);
->(() => x + v) : () => any
->() => x + v : () => any
->x + v : any
->x : any
->v : any
+>(() => x + v) : () => string
+>() => x + v : () => string
+>x + v : string
+>x : string
+>v : string
     }
 
     use(v);
 >use(v) : any
 >use : (a: any) => any
->v : any
+>v : string
 }
 
 function foo1_c(x) {