feat: resolve Svelte components using TS from exports map

This change allows people to write export maps using only a `svelte` condition (and no `types` condition) and still have the types for their components resolved (i.e. the import is found) as long as they use TypeScript (i.e. have lang="ts" attribute) inside it. This should help people using monorepo setups with strong typings and not wanting to provide d.ts files alongside.

This is achieved doing three adjustments:
- add `customConditions: ['svelte']` to the compiler options, so that TypeScript's resolution algorithm takes it into account
- ensure that Svelte files have a module kind of ESM, so that TypeScript's resolution algorithm goes into the right branches
- deal with `.d.svelte.ts` files in the context of an exports map, because that's what TypeScript will try to resolve this to in the end

This is also related to #1056 insofar that we align with TypeScript for this new capability: We don't resolve the file if it's a component not using TypeScript (i.e. not having the lang="ts" tag), similar to how TypeScript does not resolve .js files within node_modules

packages/language-server/src/plugins/typescript/module-loader.ts

@@ -104,10 +104,13 @@ class ImpliedNodeFormatResolver {
             return undefined;
         }
 
-        let mode = undefined;
+        let mode: ReturnType<typeof ts.getModeForResolutionAtIndex> = undefined;
         if (sourceFile) {
             this.cacheImpliedNodeFormat(sourceFile, compilerOptions);
             mode = ts.getModeForResolutionAtIndex(sourceFile, importIdxInFile, compilerOptions);
+            if (!mode && isSvelteFilePath(importPath)) {
+                mode = ts.ModuleKind.ESNext; // necessary for TS' module resolution to go into the right branches
+            }
         }
         return mode;
     }
@@ -293,6 +296,21 @@ export function createSvelteModuleLoader(
 
         const snapshot = getSnapshot(resolvedFileName);
 
+        // Align with TypeScript behavior: If the Svelte file is not using TypeScript,
+        // mark it as unresolved so that people need to provide a .d.ts file.
+        // For backwards compatibility we're not doing this for files from packages
+        // without an exports map, because that may break too many existing projects.
+        if (
+            resolvedModule.isExternalLibraryImport &&
+            resolvedModule.extension === '.d.svelte.ts' && // this tells us it's from an exports map
+            snapshot.scriptKind !== ts.ScriptKind.TS
+        ) {
+            return {
+                ...resolvedModuleWithFailedLookup,
+                resolvedModule: undefined
+            };
+        }
+
         const resolvedSvelteModule: ts.ResolvedModuleFull = {
             extension: getExtensionFromScriptKind(snapshot && snapshot.scriptKind),
             resolvedFileName,

packages/language-server/src/plugins/typescript/service.ts

@@ -736,6 +736,13 @@ async function createLanguageService(
             }
         }
 
+        // Necessary to be able to resolve export maps that only contain a "svelte" condition without an accompanying "types" condition
+        // https://www.typescriptlang.org/tsconfig/#customConditions
+        if (!compilerOptions.customConditions?.includes('svelte')) {
+            compilerOptions.customConditions = compilerOptions.customConditions ?? [];
+            compilerOptions.customConditions.push('svelte');
+        }
+
         const svelteConfigDiagnostics = checkSvelteInput(parsedConfig);
         if (svelteConfigDiagnostics.length > 0) {
             docContext.reportConfigError?.({

packages/language-server/src/plugins/typescript/utils.ts

@@ -74,7 +74,13 @@ export function isVirtualSvelteFilePath(filePath: string) {
 }
 
 export function toRealSvelteFilePath(filePath: string) {
-    return filePath.slice(0, -'.ts'.length);
+    filePath = filePath.slice(0, -'.ts'.length);
+    // When a .svelte file referenced inside an exports map of a package.json is tried to be resolved,
+    // TypeScript will probe for the file with a .d.svelte.ts extension.
+    if (filePath.endsWith('.d.svelte')) {
+        filePath = filePath.slice(0, -8) + 'svelte';
+    }
+    return filePath;
 }
 
 export function toVirtualSvelteFilePath(filePath: string) {

packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/expectedv2.json

@@ -0,0 +1,19 @@
+[
+    {
+        "code": 2307,
+        "message": "Cannot find module 'package/y' or its corresponding type declarations.",
+        "range": {
+            "start": {
+                "character": 38,
+                "line": 3
+            },
+            "end": {
+                "character": 49,
+                "line": 3
+            }
+        },
+        "severity": 1,
+        "source": "ts",
+        "tags": []
+    }
+]

packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/input.svelte

@@ -0,0 +1,9 @@
+<script lang="ts">
+    import DefaultSvelteWithTS from 'package';
+    import SubWithDTS from 'package/x';
+    import SubWithoutDTSAndNotTS from 'package/y';
+</script>
+
+<DefaultSvelteWithTS />
+<SubWithDTS />
+<SubWithoutDTSAndNotTS />

packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/tsconfig.json

@@ -0,0 +1,7 @@
+{
+    "compilerOptions": {
+        "module": "esnext",
+        "target": "esnext",
+        "moduleResolution": "Bundler"
+    }
+}