Type of for-in variable is keyof T when object type is a type parameter

Author: ahejlsberg

Diff for: src/compiler/checker.ts

@@ -3216,11 +3216,11 @@ namespace ts {
                 }
             }
 
-            // A variable declared in a for..in statement is always of type string
+            // A variable declared in a for..in statement is of type string, or of type keyof T when the
+            // right hand expression is of a type parameter type.
             if (declaration.parent.parent.kind === SyntaxKind.ForInStatement) {
-                // const indexType = getIndexType(checkNonNullExpression((<ForInStatement>declaration.parent.parent).expression));
-                // return indexType.flags & TypeFlags.Index ? indexType : stringType;
-                return stringType;
+                const indexType = getIndexType(checkNonNullExpression((<ForInStatement>declaration.parent.parent).expression));
+                return indexType.flags & TypeFlags.Index ? indexType : stringType;
             }
 
             if (declaration.parent.parent.kind === SyntaxKind.ForOfStatement) {