Prepare module resolution for resolving TS extensions

src/compiler/checker.ts

@@ -139,6 +139,7 @@ import {
     ElementFlags,
     EmitFlags,
     EmitHint,
+    emitModuleKindIsNonNodeESM,
     EmitResolver,
     EmitTextWriter,
     emptyArray,
@@ -458,6 +459,7 @@ import {
     isConstructorTypeNode,
     isConstTypeReference,
     isDeclaration,
+    isDeclarationFileName,
     isDeclarationName,
     isDeclarationReadonly,
     isDecorator,
@@ -897,6 +899,7 @@ import {
     setTextRangePosEnd,
     setValueDeclaration,
     ShorthandPropertyAssignment,
+    shouldAllowImportingTsExtension,
     shouldPreserveConstEnums,
     Signature,
     SignatureDeclaration,
@@ -4727,6 +4730,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 (isModuleDeclaration(location) ? location : location.parent && isModuleDeclaration(location.parent) && location.parent.name === location ? location.parent : undefined)?.name ||
                 (isLiteralImportTypeNode(location) ? location : undefined)?.argument.literal;
         const mode = contextSpecifier && isStringLiteralLike(contextSpecifier) ? getModeForUsageLocation(currentSourceFile, contextSpecifier) : currentSourceFile.impliedNodeFormat;
+        const moduleResolutionKind = getEmitModuleResolutionKind(compilerOptions);
         const resolvedModule = getResolvedModule(currentSourceFile, moduleReference, mode);
         const resolutionDiagnostic = resolvedModule && getResolutionDiagnostic(compilerOptions, resolvedModule);
         const sourceFile = resolvedModule
@@ -4737,11 +4741,28 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             if (resolutionDiagnostic) {
                 error(errorNode, resolutionDiagnostic, moduleReference, resolvedModule.resolvedFileName);
             }
+
+            if (resolvedModule.resolvedUsingTsExtension && isDeclarationFileName(moduleReference)) {
+                const importOrExport =
+                    findAncestor(location, isImportDeclaration)?.importClause ||
+                    findAncestor(location, or(isImportEqualsDeclaration, isExportDeclaration));
+                if (importOrExport && !importOrExport.isTypeOnly || findAncestor(location, isImportCall)) {
+                    error(
+                        errorNode,
+                        Diagnostics.A_declaration_file_cannot_be_imported_without_import_type_Did_you_mean_to_import_an_implementation_file_0_instead,
+                        getSuggestedImportSource(Debug.checkDefined(tryExtractTSExtension(moduleReference))));
+                }
+            }
+            else if (resolvedModule.resolvedUsingTsExtension && !shouldAllowImportingTsExtension(compilerOptions, currentSourceFile.fileName)) {
+                const tsExtension = Debug.checkDefined(tryExtractTSExtension(moduleReference));
+                error(errorNode, Diagnostics.An_import_path_can_only_end_with_a_0_extension_when_allowImportingTsExtensions_is_enabled, tsExtension);
+            }
+
             if (sourceFile.symbol) {
                 if (resolvedModule.isExternalLibraryImport && !resolutionExtensionIsTSOrJson(resolvedModule.extension)) {
                     errorOnImplicitAnyModule(/*isError*/ false, errorNode, resolvedModule, moduleReference);
                 }
-                if (getEmitModuleResolutionKind(compilerOptions) === ModuleResolutionKind.Node16 || getEmitModuleResolutionKind(compilerOptions) === ModuleResolutionKind.NodeNext) {
+                if (moduleResolutionKind === ModuleResolutionKind.Node16 || moduleResolutionKind === ModuleResolutionKind.NodeNext) {
                     const isSyncImport = (currentSourceFile.impliedNodeFormat === ModuleKind.CommonJS && !findAncestor(location, isImportCall)) || !!findAncestor(location, isImportEqualsDeclaration);
                     const overrideClauseHost = findAncestor(location, l => isImportTypeNode(l) || isExportDeclaration(l) || isImportDeclaration(l)) as ImportTypeNode | ImportDeclaration | ExportDeclaration | undefined;
                     const overrideClause = overrideClauseHost && isImportTypeNode(overrideClauseHost) ? overrideClauseHost.assertions?.assertClause : overrideClauseHost?.assertClause;
@@ -4849,25 +4870,14 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             else {
                 const tsExtension = tryExtractTSExtension(moduleReference);
                 const isExtensionlessRelativePathImport = pathIsRelative(moduleReference) && !hasExtension(moduleReference);
-                const moduleResolutionKind = getEmitModuleResolutionKind(compilerOptions);
                 const resolutionIsNode16OrNext = moduleResolutionKind === ModuleResolutionKind.Node16 ||
                     moduleResolutionKind === ModuleResolutionKind.NodeNext;
                 if (tsExtension) {
-                    const diag = Diagnostics.An_import_path_cannot_end_with_a_0_extension_Consider_importing_1_instead;
-                    const importSourceWithoutExtension = removeExtension(moduleReference, tsExtension);
-                    let replacedImportSource = importSourceWithoutExtension;
-                    /**
-                     * Direct users to import source with .js extension if outputting an ES module.
-                     * @see https://github.com/microsoft/TypeScript/issues/42151
-                     */
-                    if (moduleKind >= ModuleKind.ES2015) {
-                        replacedImportSource += tsExtension === Extension.Mts ? ".mjs" : tsExtension === Extension.Cts ? ".cjs" : ".js";
-                    }
-                    error(errorNode, diag, tsExtension, replacedImportSource);
+                    errorOnTSExtensionImport(tsExtension);
                 }
                 else if (!compilerOptions.resolveJsonModule &&
                     fileExtensionIs(moduleReference, Extension.Json) &&
-                    getEmitModuleResolutionKind(compilerOptions) !== ModuleResolutionKind.Classic &&
+                    moduleResolutionKind !== ModuleResolutionKind.Classic &&
                     hasJsonModuleEmitEnabled(compilerOptions)) {
                     error(errorNode, Diagnostics.Cannot_find_module_0_Consider_using_resolveJsonModule_to_import_module_with_json_extension, moduleReference);
                 }
@@ -4889,6 +4899,23 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             }
         }
         return undefined;
+
+        function errorOnTSExtensionImport(tsExtension: string) {
+            const diag = Diagnostics.An_import_path_cannot_end_with_a_0_extension_Consider_importing_1_instead;
+            error(errorNode, diag, tsExtension, getSuggestedImportSource(tsExtension));
+        }
+
+        function getSuggestedImportSource(tsExtension: string) {
+            const importSourceWithoutExtension = removeExtension(moduleReference, tsExtension);
+            /**
+             * Direct users to import source with .js extension if outputting an ES module.
+             * @see https://github.com/microsoft/TypeScript/issues/42151
+             */
+            if (emitModuleKindIsNonNodeESM(moduleKind) || mode === ModuleKind.ESNext) {
+                return importSourceWithoutExtension + (tsExtension === Extension.Mts ? ".mjs" : tsExtension === Extension.Cts ? ".cjs" : ".js");
+            }
+            return importSourceWithoutExtension;
+        }
     }
 
     function errorOnImplicitAnyModule(isError: boolean, errorNode: Node, { packageId, resolvedFileName }: ResolvedModuleFull, moduleReference: string): void {

src/compiler/commandLineParser.ts

@@ -1081,6 +1081,14 @@ const commandOptionsWithoutBuild: CommandLineOption[] = [
         category: Diagnostics.Modules,
         description: Diagnostics.List_of_file_name_suffixes_to_search_when_resolving_a_module,
     },
+    // {
+    //     name: "allowImportingTsExtensions",
+    //     type: "boolean",
+    //     affectsModuleResolution: true,
+    //     category: Diagnostics.Modules,
+    //     description: Diagnostics.Allow_imports_to_include_TypeScript_file_extensions_Requires_moduleResolution_minimal_and_either_noEmit_or_emitDeclarationOnly_to_be_set,
+    //     defaultValueDescription: false,
+    // },
 
     // Source Maps
     {

src/compiler/diagnosticMessages.json

@@ -3563,6 +3563,10 @@
         "category": "Error",
         "code": 2845
     },
+    "A declaration file cannot be imported without 'import type'. Did you mean to import an implementation file '{0}' instead?": {
+        "category": "Error",
+        "code": 2846
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
@@ -4221,6 +4225,14 @@
         "category": "Error",
         "code": 5095
     },
+    "Option 'allowImportingTsExtensions' can only be used when 'moduleResolution' is set to 'minimal' and either 'noEmit' or 'emitDeclarationOnly' is set.": {
+        "category": "Error",
+        "code": 5096
+    },
+    "An import path can only end with a '{0}' extension when 'allowImportingTsExtensions' is enabled.": {
+        "category": "Error",
+        "code": 5097
+    },
 
     "Generates a sourcemap for each corresponding '.d.ts' file.": {
         "category": "Message",
@@ -4443,10 +4455,6 @@
         "category": "Message",
         "code": 6066
     },
-    "Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6).": {
-        "category": "Message",
-        "code": 6069
-    },
     "Initializes a TypeScript project and creates a tsconfig.json file.": {
         "category": "Message",
         "code": 6070
@@ -5430,6 +5438,10 @@
         "category": "Message",
         "code": 6406
     },
+    "Allow imports to include TypeScript file extensions. Requires '--moduleResolution minimal' and either '--noEmit' or '--emitDeclarationOnly' to be set.": {
+        "category": "Message",
+        "code": 6407
+    },
 
     "The expected type comes from property '{0}' which is declared here on type '{1}'": {
         "category": "Message",