Improve errors on module: node12 and extensionless relative imports (#46486)

* add new error + suggestions

* push down assert defined

* remove comment

* fix esm module import detection, update baselines

* add and update new tests

* fix review comments

* remove renamed baseline references

* update node modules test baselines

* fix error message, add new tests

* update old tests with new error message

src/compiler/checker.ts

@@ -1016,6 +1016,22 @@ namespace ts {
         const builtinGlobals = createSymbolTable();
         builtinGlobals.set(undefinedSymbol.escapedName, undefinedSymbol);
 
+        // Extensions suggested for path imports when module resolution is node12 or higher.
+        // The first element of each tuple is the extension a file has.
+        // The second element of each tuple is the extension that should be used in a path import.
+        // e.g. if we want to import file `foo.mts`, we should write `import {} from "./foo.mjs".
+        const suggestedExtensions: [string, string][] = [
+            [".mts", ".mjs"],
+            [".ts", ".js"],
+            [".cts", ".cjs"],
+            [".mjs", ".mjs"],
+            [".js", ".js"],
+            [".cjs", ".cjs"],
+            [".tsx", compilerOptions.jsx === JsxEmit.Preserve ? ".jsx" : ".js"],
+            [".jsx", ".jsx"],
+            [".json", ".json"],
+        ];
+
         initializeTypeChecker();
 
         return checker;
@@ -3443,7 +3459,7 @@ namespace ts {
                     (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 resolvedModule = getResolvedModule(currentSourceFile, moduleReference, mode)!; // TODO: GH#18217
+            const resolvedModule = getResolvedModule(currentSourceFile, moduleReference, mode);
             const resolutionDiagnostic = resolvedModule && getResolutionDiagnostic(compilerOptions, resolvedModule);
             const sourceFile = resolvedModule && !resolutionDiagnostic && host.getSourceFile(resolvedModule.resolvedFileName);
             if (sourceFile) {
@@ -3489,10 +3505,10 @@ namespace ts {
             if (resolvedModule && !resolutionExtensionIsTSOrJson(resolvedModule.extension) && resolutionDiagnostic === undefined || resolutionDiagnostic === Diagnostics.Could_not_find_a_declaration_file_for_module_0_1_implicitly_has_an_any_type) {
                 if (isForAugmentation) {
                     const diag = Diagnostics.Invalid_module_name_in_augmentation_Module_0_resolves_to_an_untyped_module_at_1_which_cannot_be_augmented;
-                    error(errorNode, diag, moduleReference, resolvedModule.resolvedFileName);
+                    error(errorNode, diag, moduleReference, resolvedModule!.resolvedFileName);
                 }
                 else {
-                    errorOnImplicitAnyModule(/*isError*/ noImplicitAny && !!moduleNotFoundError, errorNode, resolvedModule, moduleReference);
+                    errorOnImplicitAnyModule(/*isError*/ noImplicitAny && !!moduleNotFoundError, errorNode, resolvedModule!, moduleReference);
                 }
                 // Failed imports and untyped modules are both treated in an untyped manner; only difference is whether we give a diagnostic first.
                 return undefined;
@@ -3513,6 +3529,10 @@ namespace ts {
                 }
                 else {
                     const tsExtension = tryExtractTSExtension(moduleReference);
+                    const isExtensionlessRelativePathImport = pathIsRelative(moduleReference) && !hasExtension(moduleReference);
+                    const moduleResolutionKind = getEmitModuleResolutionKind(compilerOptions);
+                    const resolutionIsNode12OrNext = moduleResolutionKind === ModuleResolutionKind.Node12 ||
+                        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);
@@ -3532,6 +3552,18 @@ namespace ts {
                         hasJsonModuleEmitEnabled(compilerOptions)) {
                         error(errorNode, Diagnostics.Cannot_find_module_0_Consider_using_resolveJsonModule_to_import_module_with_json_extension, moduleReference);
                     }
+                    else if (mode === ModuleKind.ESNext && resolutionIsNode12OrNext && isExtensionlessRelativePathImport) {
+                        const absoluteRef = getNormalizedAbsolutePath(moduleReference, getDirectoryPath(currentSourceFile.path));
+                        const suggestedExt = suggestedExtensions.find(([actualExt, _importExt]) => host.fileExists(absoluteRef + actualExt))?.[1];
+                        if (suggestedExt) {
+                            error(errorNode,
+                                Diagnostics.Relative_import_paths_need_explicit_file_extensions_in_EcmaScript_imports_when_moduleResolution_is_node12_or_nodenext_Did_you_mean_0,
+                                moduleReference + suggestedExt);
+                        }
+                        else {
+                            error(errorNode, Diagnostics.Relative_import_paths_need_explicit_file_extensions_in_EcmaScript_imports_when_moduleResolution_is_node12_or_nodenext_Consider_adding_an_extension_to_the_import_path);
+                        }
+                    }
                     else {
                         error(errorNode, moduleNotFoundError, moduleReference);
                     }

src/compiler/diagnosticMessages.json

@@ -3345,6 +3345,14 @@
         "category": "Error",
         "code": 2833
     },
+    "Relative import paths need explicit file extensions in EcmaScript imports when '--moduleResolution' is 'node12' or 'nodenext'. Consider adding an extension to the import path.": {
+        "category": "Error",
+        "code": 2834
+    },
+    "Relative import paths need explicit file extensions in EcmaScript imports when '--moduleResolution' is 'node12' or 'nodenext'. Did you mean '{0}'?": {
+        "category": "Error",
+        "code": 2835
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",

src/compiler/types.ts

@@ -5992,7 +5992,7 @@ namespace ts {
     export enum ModuleResolutionKind {
         Classic  = 1,
         NodeJs   = 2,
-        // Starting with node12, node's module resolver has significant departures from tranditional cjs resolution
+        // Starting with node12, node's module resolver has significant departures from traditional cjs resolution
         // to better support ecmascript modules and their use within node - more features are still being added, so
         // we can expect it to change over time, and as such, offer both a `NodeNext` moving resolution target, and a `Node12`
         // version-anchored resolution target