show error if block scoped variable declared in the loop is captured in closure

src/compiler/checker.ts

@@ -4905,10 +4905,55 @@ module ts {
 
             checkCollisionWithCapturedSuperVariable(node, node);
             checkCollisionWithCapturedThisVariable(node, node);
+            checkBlockScopedBindingCapturedInLoop(node, symbol);
 
             return getNarrowedTypeOfSymbol(getExportSymbolOfValueSymbolIfExported(symbol), node);
         }
 
+        function isNameScopeBoundary(n: Node): boolean {
+            return isAnyFunction(n) || n.kind === SyntaxKind.ModuleDeclaration || n.kind === SyntaxKind.SourceFile;
+        }
+
+        function checkBlockScopedBindingCapturedInLoop(node: Identifier, symbol: Symbol): void {
+            if (languageVersion >= ScriptTarget.ES6 || (symbol.flags & SymbolFlags.BlockScopedVariable) === 0) {
+                return;
+            }
+
+            // - check if binding is used in some function 
+            // (stop the walk when reaching container of binding declaration)
+            // - if first check succeeded - check if variable is declared inside the loop
+
+            // var decl -> var decl list -> parent
+            var container = (<VariableDeclaration>symbol.valueDeclaration).parent.parent;
+            if (container.kind === SyntaxKind.VariableStatement) {
+                container = container.parent;
+            }
+            
+            var inFunction = false;
+            var current = node.parent;
+            while (current && current !== container) {
+                if (isAnyFunction(current)) {
+                    inFunction = true;
+                    break;
+                }
+                current = current.parent;
+            }
+
+            if (!inFunction) {
+                return;
+            }
+
+            var current: Node = container;
+            while (current && !isNameScopeBoundary(current)) {
+                if (isIterationStatement(current, /*lookInLabeledStatements*/ false)) {
+                    getNodeLinks(current).flags |= NodeCheckFlags.BlockScopedBindingCapturedInLoop;
+                    grammarErrorOnFirstToken(current, Diagnostics.Code_in_the_loop_captures_block_scoped_variable_0_in_closure_This_is_natively_supported_in_ECMAScript_6_or_higher, declarationNameToString(node));
+                    break;
+                }
+                current = current.parent;
+            }
+        }
+
         function captureLexicalThis(node: Node, container: Node): void {
             var classNode = container.parent && container.parent.kind === SyntaxKind.ClassDeclaration ? container.parent : undefined;
             getNodeLinks(node).flags |= NodeCheckFlags.LexicalThis;

src/compiler/diagnosticInformationMap.generated.ts

@@ -382,6 +382,7 @@ module ts {
         Property_0_does_not_exist_on_const_enum_1: { code: 4088, category: DiagnosticCategory.Error, key: "Property '{0}' does not exist on 'const' enum '{1}'." },
         let_is_not_allowed_to_be_used_as_a_name_in_let_or_const_declarations: { code: 4089, category: DiagnosticCategory.Error, key: "'let' is not allowed to be used as a name in 'let' or 'const' declarations." },
         Cannot_initialize_outer_scoped_variable_0_in_the_same_scope_as_block_scoped_declaration_1: { code: 4090, category: DiagnosticCategory.Error, key: "Cannot initialize outer scoped variable '{0}' in the same scope as block scoped declaration '{1}'." },
+        Code_in_the_loop_captures_block_scoped_variable_0_in_closure_This_is_natively_supported_in_ECMAScript_6_or_higher: { code: 4091, category: DiagnosticCategory.Error, key: "Code in the loop captures block-scoped variable '{0}' in closure. This is natively supported in ECMAScript 6 or higher." },
         The_current_host_does_not_support_the_0_option: { code: 5001, category: DiagnosticCategory.Error, key: "The current host does not support the '{0}' option." },
         Cannot_find_the_common_subdirectory_path_for_the_input_files: { code: 5009, category: DiagnosticCategory.Error, key: "Cannot find the common subdirectory path for the input files." },
         Cannot_read_file_0_Colon_1: { code: 5012, category: DiagnosticCategory.Error, key: "Cannot read file '{0}': {1}" },

src/compiler/diagnosticMessages.json

@@ -1521,6 +1521,10 @@
         "category": "Error",
         "code": 4090
     },
+    "Code in the loop captures block-scoped variable '{0}' in closure. This is natively supported in ECMAScript 6 or higher.": {
+        "category": "Error",
+        "code": 4091
+    },
     "The current host does not support the '{0}' option.": {
         "category": "Error",
         "code": 5001

src/compiler/types.ts

@@ -1262,6 +1262,7 @@ module ts {
 
         // Values for enum members have been computed, and any errors have been reported for them.
         EnumValuesComputed  = 0x00000080,
+        BlockScopedBindingCapturedInLoop = 0x00000100,
     }
 
     export interface NodeLinks {