Have module resolution be independend of --allowJs and --jsx flags

src/compiler/checker.ts

@@ -1374,7 +1374,8 @@ namespace ts {
             }
 
             const resolvedModule = getResolvedModule(getSourceFileOfNode(location), moduleReference);
-            const sourceFile = resolvedModule && host.getSourceFile(resolvedModule.resolvedFileName);
+            const resolutionDiagnostic = resolvedModule && getResolutionDiagnostic(compilerOptions, resolvedModule);
+            const sourceFile = resolvedModule && !resolutionDiagnostic && host.getSourceFile(resolvedModule.resolvedFileName);
             if (sourceFile) {
                 if (sourceFile.symbol) {
                     // merged symbol is module declaration symbol combined with all augmentations
@@ -1396,13 +1397,18 @@ namespace ts {
 
             if (moduleNotFoundError) {
                 // report errors only if it was requested
-                const tsExtension = tryExtractTypeScriptExtension(moduleName);
-                if (tsExtension) {
-                    const diag = Diagnostics.An_import_path_cannot_end_with_a_0_extension_Consider_importing_1_instead;
-                    error(errorNode, diag, tsExtension, removeExtension(moduleName, tsExtension));
+                if (resolutionDiagnostic) {
+                    error(errorNode, resolutionDiagnostic, moduleName, resolvedModule.resolvedFileName);
                 }
                 else {
-                    error(errorNode, moduleNotFoundError, moduleName);
+                    const tsExtension = tryExtractTypeScriptExtension(moduleName);
+                    if (tsExtension) {
+                        const diag = Diagnostics.An_import_path_cannot_end_with_a_0_extension_Consider_importing_1_instead;
+                        error(errorNode, diag, tsExtension, removeExtension(moduleName, tsExtension));
+                    }
+                    else {
+                        error(errorNode, moduleNotFoundError, moduleName);
+                    }
                 }
             }
             return undefined;

src/compiler/core.ts

@@ -1902,10 +1902,6 @@ namespace ts {
         return path.substring(0, path.length - extension.length);
     }
 
-    export function isJsxOrTsxExtension(ext: string): boolean {
-        return ext === ".jsx" || ext === ".tsx";
-    }
-
     export function changeExtension<T extends string | Path>(path: T, newExtension: string): T {
         return <T>(removeFileExtension(path) + newExtension);
     }
@@ -2098,4 +2094,33 @@ namespace ts {
         //  pos === undefined || pos === null || isNaN(pos) || pos < 0;
         return !(pos >= 0);
     }
+
+    /** True if an extension is one of the supported TypeScript extensions. */
+    export function extensionIsTypeScript(ext: Extension): boolean {
+        return ext <= Extension.LastTypeScriptExtension;
+    }
+
+    /**
+     * Gets the extension from a path.
+     * Path must have a valid extension.
+     */
+    export function extensionFromPath(path: string): Extension {
+        if (fileExtensionIs(path, ".d.ts")) {
+            return Extension.Dts;
+        }
+        if (fileExtensionIs(path, ".ts")) {
+            return Extension.Ts;
+        }
+        if (fileExtensionIs(path, ".tsx")) {
+            return Extension.Tsx;
+        }
+        if (fileExtensionIs(path, ".js")) {
+            return Extension.Js;
+        }
+        if (fileExtensionIs(path, ".jsx")) {
+            return Extension.Jsx;
+        }
+        Debug.fail(`File ${path} has unknown extension.`);
+        return Extension.Js;
+    }
 }

src/compiler/diagnosticMessages.json

@@ -2693,7 +2693,7 @@
         "category": "Message",
         "code": 6099
     },
-    "'package.json' does not have 'types' field.": {
+    "'package.json' does not have a 'types' or 'main' field.": {
         "category": "Message",
         "code": 6100
     },
@@ -2841,7 +2841,7 @@
         "category": "Message",
         "code": 6136
     },
-    "No types specified in 'package.json' but 'allowJs' is set, so returning 'main' value of '{0}'": {
+    "No types specified in 'package.json', so returning 'main' value of '{0}'": {
         "category": "Message",
         "code": 6137
     },
@@ -2861,6 +2861,14 @@
         "category": "Message",
         "code": 6141
     },
+    "Module '{0}' was resolved to '{1}', but '--jsx' is not set.": {
+        "category": "Error",
+        "code": 6142
+    },
+    "Module '{0}' was resolved to '{1}', but '--allowJs' is not set.": {
+        "category": "Error",
+        "code": 6143
+    },
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
         "code": 7005