Provide better error message

Author: Andy Hanson

src/compiler/checker.ts

@@ -479,6 +479,9 @@ namespace ts {
                 }
                 recordMergedSymbol(target, source);
             }
+            else if (target.flags & SymbolFlags.NamespaceModule) {
+                error(source.valueDeclaration.name, Diagnostics.Cannot_augment_module_0_with_value_exports_because_it_resolves_to_a_non_module_entity, symbolToString(target));
+            }
             else {
                 const message = target.flags & SymbolFlags.BlockScopedVariable || source.flags & SymbolFlags.BlockScopedVariable
                     ? Diagnostics.Cannot_redeclare_block_scoped_variable_0 : Diagnostics.Duplicate_identifier_0;

src/compiler/diagnosticMessages.json

@@ -1807,6 +1807,10 @@
         "category": "Error",
         "code": 2608
     },
+    "Cannot augment module '{0}' with value exports because it resolves to a non-module entity.": {
+        "category": "Error",
+        "code": 2649
+    },
     "Cannot emit namespaced JSX elements in React": {
         "category": "Error",
         "code": 2650

tests/baselines/reference/augmentExportEquals7.errors.txt

@@ -1,22 +1,16 @@
-/node_modules/@types/lib-extender/index.d.ts(2,16): error TS2300: Duplicate identifier '"lib"'.
-/node_modules/lib/index.d.ts(1,13): error TS2300: Duplicate identifier '"lib"'.
-/node_modules/lib/index.d.ts(2,19): error TS2300: Duplicate identifier '"lib"'.
+/node_modules/@types/lib-extender/index.d.ts(2,16): error TS2649: Cannot augment module 'lib' with value exports because it resolves to a non-module entity.
 
 
-==== /node_modules/lib/index.d.ts (2 errors) ====
+==== /node_modules/lib/index.d.ts (0 errors) ====
     declare var lib: () => void;
-                ~~~
-!!! error TS2300: Duplicate identifier '"lib"'.
     declare namespace lib {}
-                      ~~~
-!!! error TS2300: Duplicate identifier '"lib"'.
     export = lib;
 
 ==== /node_modules/@types/lib-extender/index.d.ts (1 errors) ====
     import * as lib from "lib";
     declare module "lib" {
                    ~~~~~
-!!! error TS2300: Duplicate identifier '"lib"'.
+!!! error TS2649: Cannot augment module 'lib' with value exports because it resolves to a non-module entity.
         export function fn(): void;
     }