Change error message for default export in namespace

src/compiler/checker.ts

@@ -19062,7 +19062,13 @@ namespace ts {
 
             const container = node.parent.kind === SyntaxKind.SourceFile ? <SourceFile>node.parent : <ModuleDeclaration>node.parent.parent;
             if (container.kind === SyntaxKind.ModuleDeclaration && !isAmbientModule(container)) {
-                error(node, Diagnostics.An_export_assignment_cannot_be_used_in_a_namespace);
+                if (node.isExportEquals) {
+                    error(node, Diagnostics.An_export_assignment_cannot_be_used_in_a_namespace);
+                }
+                else {
+                    error(node, Diagnostics.A_default_export_can_only_be_used_in_an_ECMAScript_style_module);
+                }
+
                 return;
             }
             // Grammar checking

src/compiler/diagnosticMessages.json

@@ -855,6 +855,10 @@
         "category": "Error",
         "code": 1318
     },
+    "A default export can only be used in an ECMAScript-style module.": {
+        "category": "Error",
+        "code": 1319
+    },
     "Duplicate identifier '{0}'.": {
         "category": "Error",
         "code": 2300

tests/baselines/reference/parserExportAssignment9.errors.txt

@@ -0,0 +1,16 @@
+tests/cases/conformance/parser/ecmascript5/ExportAssignments/parserExportAssignment9.ts(2,3): error TS1319: A default export can only be used in an ECMAScript-style module.
+tests/cases/conformance/parser/ecmascript5/ExportAssignments/parserExportAssignment9.ts(6,3): error TS1319: A default export can only be used in an ECMAScript-style module.
+
+
+==== tests/cases/conformance/parser/ecmascript5/ExportAssignments/parserExportAssignment9.ts (2 errors) ====
+    namespace Foo {
+      export default foo;
+      ~~~~~~~~~~~~~~~~~~~
+!!! error TS1319: A default export can only be used in an ECMAScript-style module.
+    }
+
+    module Bar {
+      export default bar;
+      ~~~~~~~~~~~~~~~~~~~
+!!! error TS1319: A default export can only be used in an ECMAScript-style module.
+    }

tests/baselines/reference/parserExportAssignment9.js

@@ -0,0 +1,18 @@
+//// [parserExportAssignment9.ts]
+namespace Foo {
+  export default foo;
+}
+
+module Bar {
+  export default bar;
+}
+
+//// [parserExportAssignment9.js]
+var Foo;
+(function (Foo) {
+    export default foo;
+})(Foo || (Foo = {}));
+var Bar;
+(function (Bar) {
+    export default bar;
+})(Bar || (Bar = {}));

tests/cases/conformance/parser/ecmascript5/ExportAssignments/parserExportAssignment9.ts

@@ -0,0 +1,7 @@
+namespace Foo {
+  export default foo;
+}
+
+module Bar {
+  export default bar;
+}