Add specific error message for unwanted '.ts' extension

src/compiler/checker.ts

@@ -1344,7 +1344,12 @@ namespace ts {
 
             if (moduleNotFoundError) {
                 // report errors only if it was requested
-                error(moduleReferenceLiteral, moduleNotFoundError, moduleName);
+                if (hasTypeScriptFileExtensionNonDts(moduleName)) {
+                    error(moduleReferenceLiteral, Diagnostics.Module_name_should_not_include_a_ts_extension_Colon_0, moduleName);
+                }
+                else {
+                    error(moduleReferenceLiteral, moduleNotFoundError, moduleName);
+                }
             }
             return undefined;
         }

src/compiler/core.ts

@@ -1134,7 +1134,8 @@ namespace ts {
     /**
      *  List of supported extensions in order of file resolution precedence.
      */
-    export const supportedTypeScriptExtensions = [".ts", ".tsx", ".d.ts"];
+    export const supportedTypeScriptExtensionsNonDts = [".ts", ".tsx"];
+    export const supportedTypeScriptExtensions = supportedTypeScriptExtensionsNonDts.concat([".d.ts"]);
     export const supportedJavascriptExtensions = [".js", ".jsx"];
     const allSupportedExtensions  = supportedTypeScriptExtensions.concat(supportedJavascriptExtensions);
 

src/compiler/diagnosticMessages.json

@@ -1943,6 +1943,10 @@
         "category": "Error",
         "code": 2689
     },
+    "Module name should not include a '.ts' extension: '{0}'.": {
+        "category": "Error",
+        "code": 2690
+    },
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
         "code": 4000

src/compiler/utilities.ts

@@ -2726,6 +2726,10 @@ namespace ts {
         return forEach(supportedTypeScriptExtensions, extension => fileExtensionIs(fileName, extension));
     }
 
+    export function hasTypeScriptFileExtensionNonDts(fileName: string) {
+        return forEach(supportedTypeScriptExtensionsNonDts, extension => fileExtensionIs(fileName, extension));
+    }
+
     /**
      * Replace each instance of non-ascii characters by one, two, three, or four escape sequences
      * representing the UTF-8 encoding of the character, and return the expanded char code list.

tests/baselines/reference/moduleResolutionNoTs.errors.txt

@@ -1,11 +1,14 @@
-tests/cases/compiler/user.ts(1,15): error TS2307: Cannot find module './m.ts'.
+tests/cases/compiler/user.ts(4,15): error TS2690: Module name should not include a '.ts' extension: './m.ts'.
 
 
-==== tests/cases/compiler/m.ts (0 errors) ====
-    export default 0;
-
 ==== tests/cases/compiler/user.ts (1 errors) ====
+    // '.ts' extension is OK in a reference
+    ///<reference path="./m.ts"/>
+
     import x from "./m.ts";
                   ~~~~~~~~
-!!! error TS2307: Cannot find module './m.ts'.
+!!! error TS2690: Module name should not include a '.ts' extension: './m.ts'.
+
+==== tests/cases/compiler/m.ts (0 errors) ====
+    export default 0;
 

tests/baselines/reference/moduleResolutionNoTs.js

@@ -4,6 +4,9 @@
 export default 0;
 
 //// [user.ts]
+// '.ts' extension is OK in a reference
+///<reference path="./m.ts"/>
+
 import x from "./m.ts";
 
 
@@ -12,4 +15,6 @@ import x from "./m.ts";
 exports.__esModule = true;
 exports["default"] = 0;
 //// [user.js]
+// '.ts' extension is OK in a reference
+///<reference path="./m.ts"/>
 "use strict";

tests/cases/compiler/moduleResolutionNoTs.ts

@@ -2,4 +2,7 @@
 export default 0;
 
 // @filename: user.ts
+// '.ts' extension is OK in a reference
+///<reference path="./m.ts"/>
+
 import x from "./m.ts";