Error when users define "__esModule"

src/compiler/checker.ts

@@ -21909,6 +21909,10 @@ namespace ts {
                 }
             }
 
+            if (hasModifier(node.parent.parent, ModifierFlags.Export) && compilerOptions.module !== ModuleKind.ES2015) {
+                checkESModuleMarker(node.name);
+            }
+
             const checkLetConstNames = (isLet(node) || isConst(node));
 
             // 1. LexicalDeclaration : LetOrConst BindingList ;
@@ -21921,6 +21925,22 @@ namespace ts {
             return checkLetConstNames && checkGrammarNameInLetOrConstDeclarations(node.name);
         }
 
+        function checkESModuleMarker(name: Identifier | BindingPattern): boolean {
+            if (name.kind === SyntaxKind.Identifier) {
+                if (unescapeIdentifier(name.text) === "__esModule") {
+                    return grammarErrorOnNode(name, Diagnostics.Identifier_expected_esModule_is_reserved_as_a_marker_for_transpiled_ES2015_module);
+                }
+            }
+            else {
+                const elements = (<BindingPattern>name).elements;
+                for (const element of elements) {
+                    if (!isOmittedExpression(element)) {
+                        return checkESModuleMarker(element.name);
+                    }
+                }
+            }
+        }
+
         function checkGrammarNameInLetOrConstDeclarations(name: Identifier | BindingPattern): boolean {
             if (name.kind === SyntaxKind.Identifier) {
                 if ((<Identifier>name).originalKeywordKind === SyntaxKind.LetKeyword) {

src/compiler/diagnosticMessages.json

@@ -671,6 +671,10 @@
         "category": "Error",
         "code": 1215
     },
+    "Identifier expected. '__esModule' is reserved as a marker for transpiled ES2015 module.": {
+        "category": "Error",
+        "code": 1216
+    },
     "Export assignment is not supported when '--module' flag is 'system'.": {
         "category": "Error",
         "code": 1218

src/compiler/transformers/module/module.ts

@@ -835,7 +835,7 @@ namespace ts {
             const parseTreeNode = getParseTreeNode(node);
             if (parseTreeNode && !shouldAppendEsModuleMarker) {
                 // class declaration get down-level transformed to be variable statement
-                shouldAppendEsModuleMarker = (parseTreeNode.kind === SyntaxKind.VariableStatement || parseTreeNode.kind === SyntaxKind.ClassDeclaration || parseTreeNode.kind === SyntaxKind.ImportEqualsDeclaration)
+                shouldAppendEsModuleMarker = (parseTreeNode.kind === SyntaxKind.VariableStatement ||parseTreeNode.kind === SyntaxKind.ClassDeclaration || parseTreeNode.kind === SyntaxKind.ImportEqualsDeclaration)
                     && hasModifier(parseTreeNode, ModifierFlags.Export);
             }
  

tests/baselines/reference/es5-commonjs3.errors.txt

@@ -0,0 +1,10 @@
+tests/cases/compiler/es5-commonjs3.ts(3,12): error TS1216: Identifier expected. '__esModule' is reserved as a marker for transpiled ES2015 module.
+
+
+==== tests/cases/compiler/es5-commonjs3.ts (1 errors) ====
+    
+    export default "test";
+    export var __esModule = 1;
+               ~~~~~~~~~~
+!!! error TS1216: Identifier expected. '__esModule' is reserved as a marker for transpiled ES2015 module.
+    

tests/baselines/reference/es5-commonjs3.js

@@ -8,3 +8,4 @@ export var __esModule = 1;
 "use strict";
 exports.default = "test";
 exports.__esModule = 1;
+Object.defineProperty(exports, "__esModule", { value: true });

tests/baselines/reference/es5-commonjs4.errors.txt

@@ -0,0 +1,21 @@
+tests/cases/compiler/es5-commonjs4.ts(14,12): error TS1216: Identifier expected. '__esModule' is reserved as a marker for transpiled ES2015 module.
+
+
+==== tests/cases/compiler/es5-commonjs4.ts (1 errors) ====
+    
+    export default class A
+    {
+        constructor ()
+        {
+    
+        }
+    
+        public B()
+        {
+            return 42;
+        }
+    }
+    export var __esModule = 1;
+               ~~~~~~~~~~
+!!! error TS1216: Identifier expected. '__esModule' is reserved as a marker for transpiled ES2015 module.
+    

tests/baselines/reference/es5-commonjs4.js

@@ -27,3 +27,4 @@ var A = (function () {
 }());
 exports.default = A;
 exports.__esModule = 1;
+Object.defineProperty(exports, "__esModule", { value: true });