From 1f8b5a04bfa3b4991d02eb8b8ed04c4229244de9 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Wed, 28 Aug 2024 17:39:22 +0200
Subject: [PATCH 1/9] 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
---
 .../src/plugins/typescript/module-loader.ts   | 20 ++++++++++++++++++-
 .../src/plugins/typescript/service.ts         |  7 +++++++
 .../src/plugins/typescript/utils.ts           |  8 +++++++-
 .../exports-map-svelte.only/expectedv2.json   | 19 ++++++++++++++++++
 .../exports-map-svelte.only/input.svelte      |  9 +++++++++
 .../exports-map-svelte.only/tsconfig.json     |  7 +++++++
 6 files changed, 68 insertions(+), 2 deletions(-)
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/expectedv2.json
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/input.svelte
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/tsconfig.json

diff --git a/packages/language-server/src/plugins/typescript/module-loader.ts b/packages/language-server/src/plugins/typescript/module-loader.ts
index 915e50b26..5c440dbf1 100644
--- a/packages/language-server/src/plugins/typescript/module-loader.ts
+++ b/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,
diff --git a/packages/language-server/src/plugins/typescript/service.ts b/packages/language-server/src/plugins/typescript/service.ts
index 8f5720bb0..f94e8e6ff 100644
--- a/packages/language-server/src/plugins/typescript/service.ts
+++ b/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?.({
diff --git a/packages/language-server/src/plugins/typescript/utils.ts b/packages/language-server/src/plugins/typescript/utils.ts
index 62fa6359d..9cb8b7f0f 100644
--- a/packages/language-server/src/plugins/typescript/utils.ts
+++ b/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) {
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/expectedv2.json b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/expectedv2.json
new file mode 100644
index 000000000..001dff42d
--- /dev/null
+++ b/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": []
+    }
+]
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/input.svelte b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/input.svelte
new file mode 100644
index 000000000..3a6782e1b
--- /dev/null
+++ b/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 />
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/tsconfig.json b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/tsconfig.json
new file mode 100644
index 000000000..d5e498207
--- /dev/null
+++ b/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"
+    }
+}

From cb7989c5def7a57b4d06b89f9637282cf159c021 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Wed, 28 Aug 2024 17:52:46 +0200
Subject: [PATCH 2/9] oops

---
 .../expectedv2.json                                         | 0
 .../input.svelte                                            | 0
 .../tsconfig.json                                           | 0
 .../language-server/test/plugins/typescript/service.test.ts | 6 ++++--
 4 files changed, 4 insertions(+), 2 deletions(-)
 rename packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/{exports-map-svelte.only => exports-map-svelte}/expectedv2.json (100%)
 rename packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/{exports-map-svelte.only => exports-map-svelte}/input.svelte (100%)
 rename packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/{exports-map-svelte.only => exports-map-svelte}/tsconfig.json (100%)

diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/expectedv2.json b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/expectedv2.json
similarity index 100%
rename from packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/expectedv2.json
rename to packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/expectedv2.json
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/input.svelte b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/input.svelte
similarity index 100%
rename from packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/input.svelte
rename to packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/input.svelte
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/tsconfig.json b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/tsconfig.json
similarity index 100%
rename from packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte.only/tsconfig.json
rename to packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/tsconfig.json
diff --git a/packages/language-server/test/plugins/typescript/service.test.ts b/packages/language-server/test/plugins/typescript/service.test.ts
index 50c793c6f..266496dc7 100644
--- a/packages/language-server/test/plugins/typescript/service.test.ts
+++ b/packages/language-server/test/plugins/typescript/service.test.ts
@@ -75,7 +75,8 @@ describe('service', () => {
             strict: true,
             module: ts.ModuleKind.ESNext,
             moduleResolution: ts.ModuleResolutionKind.Node10,
-            target: ts.ScriptTarget.ESNext
+            target: ts.ScriptTarget.ESNext,
+            customConditions: ['svelte']
         });
     });
 
@@ -185,7 +186,8 @@ describe('service', () => {
             moduleResolution: ts.ModuleResolutionKind.Node10,
             noEmit: true,
             skipLibCheck: true,
-            target: ts.ScriptTarget.ESNext
+            target: ts.ScriptTarget.ESNext,
+            customConditions: ['svelte']
         });
     });
 

From 4a4645ba9f6aa0da3cc8ff410237aa6dce844fc6 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Wed, 28 Aug 2024 21:16:46 +0200
Subject: [PATCH 3/9] drive-by

---
 .../diagnostics/fixtures/bindings-two-way-check/expectedv2.json  | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/bindings-two-way-check/expectedv2.json

diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/bindings-two-way-check/expectedv2.json b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/bindings-two-way-check/expectedv2.json
new file mode 100644
index 000000000..fe51488c7
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/bindings-two-way-check/expectedv2.json
@@ -0,0 +1 @@
+[]

From c138c983e5c193b9d5839f20213025814d825191 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Wed, 28 Aug 2024 21:17:23 +0200
Subject: [PATCH 4/9] drive-by

---
 .../bindings-two-way-check/expectedv2.json    | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/bindings-two-way-check/expectedv2.json b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/bindings-two-way-check/expectedv2.json
index fe51488c7..f8c8cb449 100644
--- a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/bindings-two-way-check/expectedv2.json
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/bindings-two-way-check/expectedv2.json
@@ -1 +1,18 @@
-[]
+[
+    {
+        "code": -1,
+        "message": "Unexpected token",
+        "range": {
+            "end": {
+                "character": 47,
+                "line": 12
+            },
+            "start": {
+                "character": 47,
+                "line": 12
+            }
+        },
+        "severity": 1,
+        "source": "ts"
+    }
+]

From dcc0507c7f214874b9d83155f2d64d3e5c61babb Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Thu, 29 Aug 2024 10:24:32 +0200
Subject: [PATCH 5/9] commit node_modules dir of test

---
 packages/language-server/.gitignore              |  1 +
 .../node_modules/package/foo.svelte              |  3 +++
 .../node_modules/package/package.json            | 16 ++++++++++++++++
 .../node_modules/package/x-types.d.ts            |  2 ++
 .../node_modules/package/x.svelte                |  3 +++
 .../node_modules/package/y.svelte                |  3 +++
 6 files changed, 28 insertions(+)
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/foo.svelte
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/package.json
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/x-types.d.ts
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/x.svelte
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/y.svelte

diff --git a/packages/language-server/.gitignore b/packages/language-server/.gitignore
index 5dbbc7fbb..02c400bc6 100644
--- a/packages/language-server/.gitignore
+++ b/packages/language-server/.gitignore
@@ -1,3 +1,4 @@
 dist/
 .vscode/
 node_modules/
+!test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/
\ No newline at end of file
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/foo.svelte b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/foo.svelte
new file mode 100644
index 000000000..e29d29c54
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/foo.svelte
@@ -0,0 +1,3 @@
+<script lang="ts">
+    export let foo = true;
+</script>
\ No newline at end of file
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/package.json b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/package.json
new file mode 100644
index 000000000..422ee721e
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/package.json
@@ -0,0 +1,16 @@
+{
+    "name": "package",
+    "version": "1.0.0",
+    "exports": {
+        ".": {
+            "svelte": "./foo.svelte"
+        },
+        "./x": {
+            "types": "./x-types.d.ts",
+            "svelte": "./x.svelte"
+        },
+        "./y": {
+            "svelte": "./y.svelte"
+        }
+    }
+}
\ No newline at end of file
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/x-types.d.ts b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/x-types.d.ts
new file mode 100644
index 000000000..4d27ef5b0
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/x-types.d.ts
@@ -0,0 +1,2 @@
+declare const X: any;
+export default X;
\ No newline at end of file
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/x.svelte b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/x.svelte
new file mode 100644
index 000000000..fb5dbfefa
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/x.svelte
@@ -0,0 +1,3 @@
+<script>
+    export let x = true;
+</script>
\ No newline at end of file
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/y.svelte b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/y.svelte
new file mode 100644
index 000000000..78f01008c
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/exports-map-svelte/node_modules/package/y.svelte
@@ -0,0 +1,3 @@
+<script>
+    export let y = true;
+</script>
\ No newline at end of file

From 72e815bace49c69696c69f34f06d552e28b88888 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Thu, 29 Aug 2024 23:20:11 +0200
Subject: [PATCH 6/9] use .d.svelte.ts for intercepting svelte files, which
 allows us to make `./foo.svelte` make .svelte query before .svelte.ts

---
 packages/language-server/package.json         |  2 +-
 .../src/plugins/typescript/module-loader.ts   |  3 ---
 .../src/plugins/typescript/svelte-sys.ts      | 20 +++++++++++++++----
 .../src/plugins/typescript/utils.ts           | 16 ++++++---------
 .../fixtures/import-precedence/a.svelte       |  3 +++
 .../fixtures/import-precedence/a.svelte.d.ts  |  1 +
 .../fixtures/import-precedence/b.svelte       |  3 +++
 .../fixtures/import-precedence/b.svelte.ts    |  1 +
 .../fixtures/import-precedence/c.d.svelte.ts  |  1 +
 .../fixtures/import-precedence/c.svelte       |  3 +++
 .../fixtures/import-precedence/d.svelte       |  3 +++
 .../fixtures/import-precedence/d.svelte.js    |  1 +
 .../import-precedence/expectedv2.json         | 19 ++++++++++++++++++
 .../fixtures/import-precedence/input.svelte   | 15 ++++++++++++++
 .../plugins/typescript/module-loader.test.ts  |  4 ++--
 .../plugins/typescript/svelte-sys.test.ts     | 14 +++++++++----
 16 files changed, 85 insertions(+), 24 deletions(-)
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/a.svelte
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/a.svelte.d.ts
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/b.svelte
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/b.svelte.ts
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/c.d.svelte.ts
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/c.svelte
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/d.svelte
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/d.svelte.js
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/expectedv2.json
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/input.svelte

diff --git a/packages/language-server/package.json b/packages/language-server/package.json
index 8ee68b6ed..f3702c47b 100644
--- a/packages/language-server/package.json
+++ b/packages/language-server/package.json
@@ -10,7 +10,7 @@
         "./bin/server.js": "./bin/server.js"
     },
     "scripts": {
-        "test": "cross-env TS_NODE_TRANSPILE_ONLY=true mocha --require ts-node/register \"test/**/*.ts\" --exclude \"test/**/*.d.ts\"",
+        "test": "cross-env TS_NODE_TRANSPILE_ONLY=true mocha --require ts-node/register \"test/**/*.test.ts\"",
         "build": "tsc",
         "prepublishOnly": "npm run build",
         "watch": "tsc -w"
diff --git a/packages/language-server/src/plugins/typescript/module-loader.ts b/packages/language-server/src/plugins/typescript/module-loader.ts
index 5c440dbf1..b0a3fa5f8 100644
--- a/packages/language-server/src/plugins/typescript/module-loader.ts
+++ b/packages/language-server/src/plugins/typescript/module-loader.ts
@@ -108,9 +108,6 @@ class ImpliedNodeFormatResolver {
         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;
     }
diff --git a/packages/language-server/src/plugins/typescript/svelte-sys.ts b/packages/language-server/src/plugins/typescript/svelte-sys.ts
index a639b2a58..553bd17a3 100644
--- a/packages/language-server/src/plugins/typescript/svelte-sys.ts
+++ b/packages/language-server/src/plugins/typescript/svelte-sys.ts
@@ -11,6 +11,17 @@ export function createSvelteSys(tsSystem: ts.System) {
     function svelteFileExists(path: string) {
         if (isVirtualSvelteFilePath(path)) {
             const sveltePath = toRealSvelteFilePath(path);
+
+            // First check if there's a `.svelte.d.ts` or `.d.svelte.ts` file, which should take precedence
+            const dtsPath = sveltePath.slice(0, -7) + '.svelte.d.ts';
+            const dtsPathExists = fileExistsCache.get(dtsPath) ?? tsSystem.fileExists(dtsPath);
+            fileExistsCache.set(dtsPath, dtsPathExists);
+            if (dtsPathExists) return false;
+
+            const svelteDtsPathExists = fileExistsCache.get(path) ?? tsSystem.fileExists(path);
+            fileExistsCache.set(path, svelteDtsPathExists);
+            if (svelteDtsPathExists) return false;
+
             const sveltePathExists =
                 fileExistsCache.get(sveltePath) ?? tsSystem.fileExists(sveltePath);
             fileExistsCache.set(sveltePath, sveltePathExists);
@@ -33,10 +44,11 @@ export function createSvelteSys(tsSystem: ts.System) {
         svelteFileExists,
         getRealSveltePathIfExists,
         fileExists(path: string) {
-            // We need to check both .svelte and .svelte.ts/js because that's how Svelte 5 will likely mark files with runes in them
+            // We need to check if this is a virtual svelte file
             const sveltePathExists = svelteFileExists(path);
-            const exists =
-                sveltePathExists || (fileExistsCache.get(path) ?? tsSystem.fileExists(path));
+            if (sveltePathExists) return true;
+
+            const exists = fileExistsCache.get(path) ?? tsSystem.fileExists(path);
             fileExistsCache.set(path, exists);
             return exists;
         },
@@ -66,7 +78,7 @@ export function createSvelteSys(tsSystem: ts.System) {
         const realpath = tsSystem.realpath;
         svelteSys.realpath = function (path) {
             if (svelteFileExists(path)) {
-                return realpath(toRealSvelteFilePath(path)) + '.ts';
+                return realpath(toRealSvelteFilePath(path));
             }
             return realpath(path);
         };
diff --git a/packages/language-server/src/plugins/typescript/utils.ts b/packages/language-server/src/plugins/typescript/utils.ts
index 9cb8b7f0f..a8a6ce1f6 100644
--- a/packages/language-server/src/plugins/typescript/utils.ts
+++ b/packages/language-server/src/plugins/typescript/utils.ts
@@ -70,21 +70,17 @@ export function isSvelteFilePath(filePath: string) {
 }
 
 export function isVirtualSvelteFilePath(filePath: string) {
-    return filePath.endsWith('.svelte.ts');
+    return filePath.endsWith('.d.svelte.ts');
 }
 
 export function toRealSvelteFilePath(filePath: string) {
-    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;
+    return filePath.slice(0, -11 /* 'd.svelte.ts'.length */) + 'svelte';
 }
 
-export function toVirtualSvelteFilePath(filePath: string) {
-    return filePath.endsWith('.ts') ? filePath : filePath + '.ts';
+export function toVirtualSvelteFilePath(svelteFilePath: string) {
+    return isVirtualSvelteFilePath(svelteFilePath)
+        ? svelteFilePath
+        : svelteFilePath.slice(0, -6 /* 'svelte'.length */) + 'd.svelte.ts';
 }
 
 export function ensureRealSvelteFilePath(filePath: string) {
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/a.svelte b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/a.svelte
new file mode 100644
index 000000000..1f6a1cbe4
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/a.svelte
@@ -0,0 +1,3 @@
+<script>
+    export let foo = true;
+</script>
\ No newline at end of file
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/a.svelte.d.ts b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/a.svelte.d.ts
new file mode 100644
index 000000000..227cc1bb1
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/a.svelte.d.ts
@@ -0,0 +1 @@
+export declare const a: boolean;
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/b.svelte b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/b.svelte
new file mode 100644
index 000000000..1f6a1cbe4
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/b.svelte
@@ -0,0 +1,3 @@
+<script>
+    export let foo = true;
+</script>
\ No newline at end of file
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/b.svelte.ts b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/b.svelte.ts
new file mode 100644
index 000000000..1d9db10ef
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/b.svelte.ts
@@ -0,0 +1 @@
+export const b = true;
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/c.d.svelte.ts b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/c.d.svelte.ts
new file mode 100644
index 000000000..a12f971dd
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/c.d.svelte.ts
@@ -0,0 +1 @@
+export declare const c: boolean;
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/c.svelte b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/c.svelte
new file mode 100644
index 000000000..1f6a1cbe4
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/c.svelte
@@ -0,0 +1,3 @@
+<script>
+    export let foo = true;
+</script>
\ No newline at end of file
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/d.svelte b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/d.svelte
new file mode 100644
index 000000000..1f6a1cbe4
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/d.svelte
@@ -0,0 +1,3 @@
+<script>
+    export let foo = true;
+</script>
\ No newline at end of file
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/d.svelte.js b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/d.svelte.js
new file mode 100644
index 000000000..ecee2a83c
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/d.svelte.js
@@ -0,0 +1 @@
+export const d = true;
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/expectedv2.json b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/expectedv2.json
new file mode 100644
index 000000000..357f5c02f
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/expectedv2.json
@@ -0,0 +1,19 @@
+[
+    {
+        "code": 6263,
+        "message": "Module './c.svelte' was resolved to 'c:/repos/svelte/language-tools/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/c.d.svelte.ts', but '--allowArbitraryExtensions' is not set.",
+        "range": {
+            "end": {
+                "character": 34,
+                "line": 4
+            },
+            "start": {
+                "character": 22,
+                "line": 4
+            }
+        },
+        "severity": 1,
+        "source": "ts",
+        "tags": []
+    }
+]
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/input.svelte b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/input.svelte
new file mode 100644
index 000000000..9966c707e
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/input.svelte
@@ -0,0 +1,15 @@
+<script lang="ts">
+    import { a } from './a.svelte';
+    import B from './b.svelte';
+    import { b } from './b.svelte.js';
+    import { c } from './c.svelte';
+    import D from './d.svelte';
+    import { d } from './d.svelte.js';
+    a;
+    b;
+    c;
+    d;
+</script>
+
+<B />
+<D />
diff --git a/packages/language-server/test/plugins/typescript/module-loader.test.ts b/packages/language-server/test/plugins/typescript/module-loader.test.ts
index 559eeb447..1c3561cc2 100644
--- a/packages/language-server/test/plugins/typescript/module-loader.test.ts
+++ b/packages/language-server/test/plugins/typescript/module-loader.test.ts
@@ -59,12 +59,12 @@ describe('createSvelteModuleLoader', () => {
     it('uses svelte script kind if resolved module is svelte file', async () => {
         const resolvedModule: ts.ResolvedModuleFull = {
             extension: ts.Extension.Ts,
-            resolvedFileName: 'filename.svelte.ts'
+            resolvedFileName: 'filename.d.svelte.ts'
         };
         const { getSvelteSnapshotStub, moduleResolver, svelteSys } = setup(resolvedModule);
 
         svelteSys.getRealSveltePathIfExists = (filename: string) =>
-            filename === 'filename.svelte.ts' ? 'filename.svelte' : filename;
+            filename === 'filename.d.svelte.ts' ? 'filename.svelte' : filename;
 
         const result = moduleResolver.resolveModuleNames(
             ['./normal.ts'],
diff --git a/packages/language-server/test/plugins/typescript/svelte-sys.test.ts b/packages/language-server/test/plugins/typescript/svelte-sys.test.ts
index 2da35db20..ba9a21d5a 100644
--- a/packages/language-server/test/plugins/typescript/svelte-sys.test.ts
+++ b/packages/language-server/test/plugins/typescript/svelte-sys.test.ts
@@ -29,18 +29,24 @@ describe('Svelte Sys', () => {
     }
 
     describe('#fileExists', () => {
-        it('should leave files with no .svelte.ts-ending as is', async () => {
+        it('should leave files with no .d.svelte.ts-ending as is', async () => {
             const { loader, fileExistsStub } = setupLoader();
             loader.fileExists('../file.ts');
 
             assert.strictEqual(fileExistsStub.getCall(0).args[0], '../file.ts');
         });
 
-        it('should convert .svelte.ts-endings', async () => {
+        it('should convert .d.svelte.ts-endings', async () => {
             const { loader, fileExistsStub } = setupLoader();
-            loader.fileExists('../file.svelte.ts');
+            fileExistsStub.onCall(0).returns(false);
+            fileExistsStub.onCall(1).returns(false);
+            fileExistsStub.onCall(2).returns(true);
 
-            assert.strictEqual(fileExistsStub.getCall(0).args[0], '../file.svelte');
+            loader.fileExists('../file.d.svelte.ts');
+
+            assert.strictEqual(fileExistsStub.getCall(0).args[0], '../file.svelte.d.ts');
+            assert.strictEqual(fileExistsStub.getCall(1).args[0], '../file.d.svelte.ts');
+            assert.strictEqual(fileExistsStub.getCall(2).args[0], '../file.svelte');
         });
     });
 });

From 9f75ed509e1f60e6ddbcb67df28c318653d845f3 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Thu, 29 Aug 2024 23:33:30 +0200
Subject: [PATCH 7/9] same for ts-plugin (major version bump)

---
 packages/typescript-plugin/package.json       |  2 +-
 .../typescript-plugin/src/module-loader.ts    | 52 ++++++++-----------
 packages/typescript-plugin/src/svelte-sys.ts  |  2 +-
 packages/typescript-plugin/src/utils.ts       | 10 +++-
 4 files changed, 32 insertions(+), 34 deletions(-)

diff --git a/packages/typescript-plugin/package.json b/packages/typescript-plugin/package.json
index 6928186a4..766ac42c5 100644
--- a/packages/typescript-plugin/package.json
+++ b/packages/typescript-plugin/package.json
@@ -1,6 +1,6 @@
 {
     "name": "typescript-svelte-plugin",
-    "version": "0.2.0",
+    "version": "0.3.0",
     "description": "A TypeScript Plugin providing Svelte intellisense",
     "main": "dist/src/index.js",
     "scripts": {
diff --git a/packages/typescript-plugin/src/module-loader.ts b/packages/typescript-plugin/src/module-loader.ts
index cc2003cba..76e3c1944 100644
--- a/packages/typescript-plugin/src/module-loader.ts
+++ b/packages/typescript-plugin/src/module-loader.ts
@@ -3,30 +3,7 @@ import { ConfigManager } from './config-manager';
 import { Logger } from './logger';
 import { SvelteSnapshotManager } from './svelte-snapshots';
 import { createSvelteSys } from './svelte-sys';
-import { ensureRealSvelteFilePath, isVirtualSvelteFilePath } from './utils';
-
-// TODO remove when we update to typescript 5.0
-declare module 'typescript/lib/tsserverlibrary' {
-    interface LanguageServiceHost {
-        /** @deprecated supply resolveModuleNameLiterals instead for resolution that can handle newer resolution modes like nodenext */
-        resolveModuleNames?(
-            moduleNames: string[],
-            containingFile: string,
-            reusedNames: string[] | undefined,
-            redirectedReference: ts.ResolvedProjectReference | undefined,
-            options: ts.CompilerOptions,
-            containingSourceFile?: ts.SourceFile
-        ): (ts.ResolvedModule | undefined)[];
-        resolveModuleNameLiterals?(
-            moduleLiterals: readonly ts.StringLiteralLike[],
-            containingFile: string,
-            redirectedReference: ts.ResolvedProjectReference | undefined,
-            options: ts.CompilerOptions,
-            containingSourceFile: ts.SourceFile,
-            reusedNames: readonly ts.StringLiteralLike[] | undefined
-        ): readonly ts.ResolvedModuleWithFailedLookupLocations[];
-    }
-}
+import { ensureRealSvelteFilePath, isSvelteFilePath, isVirtualSvelteFilePath } from './utils';
 
 /**
  * Caches resolved modules.
@@ -161,12 +138,22 @@ export function patchModuleLoader(
 
         return resolved.map((tsResolvedModule, idx) => {
             const moduleName = moduleNames[idx];
-            if (tsResolvedModule || !ensureRealSvelteFilePath(moduleName).endsWith('.svelte')) {
+            if (
+                !isSvelteFilePath(moduleName) ||
+                // corresponding .d.ts files take precedence over .svelte files
+                tsResolvedModule?.resolvedFileName.endsWith('.d.ts') ||
+                tsResolvedModule?.resolvedFileName.endsWith('.d.svelte.ts')
+            ) {
                 return tsResolvedModule;
             }
 
-            return resolveSvelteModuleNameFromCache(moduleName, containingFile, compilerOptions)
-                .resolvedModule;
+            const result = resolveSvelteModuleNameFromCache(
+                moduleName,
+                containingFile,
+                compilerOptions
+            ).resolvedModule;
+            // .svelte takes precedence over .svelte.ts etc
+            return result ?? tsResolvedModule;
         });
     }
 
@@ -238,14 +225,19 @@ export function patchModuleLoader(
 
         return resolved.map((tsResolvedModule, idx) => {
             const moduleName = moduleLiterals[idx].text;
+
             if (
-                tsResolvedModule.resolvedModule ||
-                !ensureRealSvelteFilePath(moduleName).endsWith('.svelte')
+                !isSvelteFilePath(moduleName) ||
+                // corresponding .d.ts files take precedence over .svelte files
+                tsResolvedModule?.resolvedModule?.resolvedFileName.endsWith('.d.ts') ||
+                tsResolvedModule?.resolvedModule?.resolvedFileName.endsWith('.d.svelte.ts')
             ) {
                 return tsResolvedModule;
             }
 
-            return resolveSvelteModuleNameFromCache(moduleName, containingFile, options);
+            const result = resolveSvelteModuleNameFromCache(moduleName, containingFile, options);
+            // .svelte takes precedence over .svelte.ts etc
+            return result ?? tsResolvedModule;
         });
     }
 
diff --git a/packages/typescript-plugin/src/svelte-sys.ts b/packages/typescript-plugin/src/svelte-sys.ts
index d530e0b47..75b0a6f80 100644
--- a/packages/typescript-plugin/src/svelte-sys.ts
+++ b/packages/typescript-plugin/src/svelte-sys.ts
@@ -30,7 +30,7 @@ export function createSvelteSys(ts: _ts, logger: Logger) {
         const realpath = ts.sys.realpath;
         svelteSys.realpath = function (path) {
             if (isVirtualSvelteFilePath(path)) {
-                return realpath(toRealSvelteFilePath(path)) + '.ts';
+                return realpath(toRealSvelteFilePath(path));
             }
             return realpath(path);
         };
diff --git a/packages/typescript-plugin/src/utils.ts b/packages/typescript-plugin/src/utils.ts
index d189f7ad1..1b14a6dbe 100644
--- a/packages/typescript-plugin/src/utils.ts
+++ b/packages/typescript-plugin/src/utils.ts
@@ -8,11 +8,17 @@ export function isSvelteFilePath(filePath: string) {
 }
 
 export function isVirtualSvelteFilePath(filePath: string) {
-    return filePath.endsWith('.svelte.ts');
+    return filePath.endsWith('.d.svelte.ts');
 }
 
 export function toRealSvelteFilePath(filePath: string) {
-    return filePath.slice(0, -'.ts'.length);
+    return filePath.slice(0, -11 /* 'd.svelte.ts'.length */) + 'svelte';
+}
+
+export function toVirtualSvelteFilePath(svelteFilePath: string) {
+    return isVirtualSvelteFilePath(svelteFilePath)
+        ? svelteFilePath
+        : svelteFilePath.slice(0, -6 /* 'svelte'.length */) + 'd.svelte.ts';
 }
 
 export function ensureRealSvelteFilePath(filePath: string) {

From 2f390ac9153d32b72f28929932662ca5f74f4a84 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Thu, 29 Aug 2024 23:41:40 +0200
Subject: [PATCH 8/9] fix test

---
 .../import-precedence/expectedv2.json         | 20 +------------------
 .../fixtures/import-precedence/tsconfig.json  | 13 ++++++++++++
 2 files changed, 14 insertions(+), 19 deletions(-)
 create mode 100644 packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/tsconfig.json

diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/expectedv2.json b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/expectedv2.json
index 357f5c02f..fe51488c7 100644
--- a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/expectedv2.json
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/expectedv2.json
@@ -1,19 +1 @@
-[
-    {
-        "code": 6263,
-        "message": "Module './c.svelte' was resolved to 'c:/repos/svelte/language-tools/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/c.d.svelte.ts', but '--allowArbitraryExtensions' is not set.",
-        "range": {
-            "end": {
-                "character": 34,
-                "line": 4
-            },
-            "start": {
-                "character": 22,
-                "line": 4
-            }
-        },
-        "severity": 1,
-        "source": "ts",
-        "tags": []
-    }
-]
+[]
diff --git a/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/tsconfig.json b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/tsconfig.json
new file mode 100644
index 000000000..eef515315
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript/features/diagnostics/fixtures/import-precedence/tsconfig.json
@@ -0,0 +1,13 @@
+{
+    "compilerOptions": {
+        "strict": true,
+        "allowJs": true,
+        "target": "ESNext",
+        /**
+          This is actually not needed, but makes the tests faster
+          because TS does not look up other types.
+        */
+        "types": ["svelte"],
+        "allowArbitraryExtensions": true // else .d.svelte.ts will be an error
+    }
+}

From ab81ce0e5c6364f8ecd516b6f98bc4b245913f34 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Fri, 30 Aug 2024 14:45:11 +0200
Subject: [PATCH 9/9] (WIP) package JSON exports support in TS

---
 .../src/plugins/typescript/module-loader.ts   |  1 +
 .../typescript-plugin/src/module-loader.ts    | 53 ++++++++++++++-----
 2 files changed, 42 insertions(+), 12 deletions(-)

diff --git a/packages/language-server/src/plugins/typescript/module-loader.ts b/packages/language-server/src/plugins/typescript/module-loader.ts
index b0a3fa5f8..86080dc15 100644
--- a/packages/language-server/src/plugins/typescript/module-loader.ts
+++ b/packages/language-server/src/plugins/typescript/module-loader.ts
@@ -299,6 +299,7 @@ export function createSvelteModuleLoader(
         // without an exports map, because that may break too many existing projects.
         if (
             resolvedModule.isExternalLibraryImport &&
+            // TODO check what happens if this resolves to a real .d.svelte.ts file
             resolvedModule.extension === '.d.svelte.ts' && // this tells us it's from an exports map
             snapshot.scriptKind !== ts.ScriptKind.TS
         ) {
diff --git a/packages/typescript-plugin/src/module-loader.ts b/packages/typescript-plugin/src/module-loader.ts
index 76e3c1944..3391b4439 100644
--- a/packages/typescript-plugin/src/module-loader.ts
+++ b/packages/typescript-plugin/src/module-loader.ts
@@ -11,12 +11,12 @@ import { ensureRealSvelteFilePath, isSvelteFilePath, isVirtualSvelteFilePath } f
 class ModuleResolutionCache {
     constructor(private readonly projectService: ts.server.ProjectService) {}
 
-    private cache = new Map<string, ts.ResolvedModuleFull>();
+    private cache = new Map<string, ts.ResolvedModuleFull | null>();
 
     /**
      * Tries to get a cached module.
      */
-    get(moduleName: string, containingFile: string): ts.ResolvedModuleFull | undefined {
+    get(moduleName: string, containingFile: string): ts.ResolvedModuleFull | null | undefined {
         return this.cache.get(this.getKey(moduleName, containingFile));
     }
 
@@ -28,10 +28,14 @@ class ModuleResolutionCache {
         containingFile: string,
         resolvedModule: ts.ResolvedModuleFull | undefined
     ) {
-        if (!resolvedModule) {
+        if (!resolvedModule && moduleName[0] === '.') {
+            // We cache unresolved modules for non-relative imports, too, because it's very likely that they don't change
+            // and we don't want to resolve them every time. If they do change, the original resolution mode will notice
+            // most of the time, and the only time this would result in a stale cache entry is if a node_modules package
+            // is added with a "svelte" condition and no "types" condition, which is rare enough.
             return;
         }
-        this.cache.set(this.getKey(moduleName, containingFile), resolvedModule);
+        this.cache.set(this.getKey(moduleName, containingFile), resolvedModule ?? null);
     }
 
     /**
@@ -42,6 +46,7 @@ class ModuleResolutionCache {
         resolvedModuleName = this.projectService.toCanonicalFileName(resolvedModuleName);
         this.cache.forEach((val, key) => {
             if (
+                val &&
                 this.projectService.toCanonicalFileName(val.resolvedFileName) === resolvedModuleName
             ) {
                 this.cache.delete(key);
@@ -87,6 +92,8 @@ export function patchModuleLoader(
     if (lsHost.resolveModuleNameLiterals) {
         lsHost.resolveModuleNameLiterals = resolveModuleNameLiterals;
     } else {
+        // TODO do we need to keep this around? We're requiring 5.0 now, so TS doesn't need it,
+        // but would this break when other TS plugins are used and we no longer provide it?
         lsHost.resolveModuleNames = resolveModuleNames;
     }
 
@@ -139,7 +146,9 @@ export function patchModuleLoader(
         return resolved.map((tsResolvedModule, idx) => {
             const moduleName = moduleNames[idx];
             if (
-                !isSvelteFilePath(moduleName) ||
+                // Only recheck relative Svelte imports or unresolved non-relative paths (which hint at node_modules,
+                // where an exports map with "svelte" but not "types" could be present)
+                (!isSvelteFilePath(moduleName) && (moduleName[0] === '.' || tsResolvedModule)) ||
                 // corresponding .d.ts files take precedence over .svelte files
                 tsResolvedModule?.resolvedFileName.endsWith('.d.ts') ||
                 tsResolvedModule?.resolvedFileName.endsWith('.d.svelte.ts')
@@ -165,7 +174,8 @@ export function patchModuleLoader(
         const svelteResolvedModule = typescript.resolveModuleName(
             name,
             containingFile,
-            compilerOptions,
+            // customConditions makes the TS algorithm look at the "svelte" condition in exports maps
+            { ...compilerOptions, customConditions: ['svelte'] },
             svelteSys
             // don't set mode or else .svelte imports couldn't be resolved
         ).resolvedModule;
@@ -225,19 +235,22 @@ export function patchModuleLoader(
 
         return resolved.map((tsResolvedModule, idx) => {
             const moduleName = moduleLiterals[idx].text;
+            const resolvedModule = tsResolvedModule.resolvedModule;
 
             if (
-                !isSvelteFilePath(moduleName) ||
+                // Only recheck relative Svelte imports or unresolved non-relative paths (which hint at node_modules,
+                // where an exports map with "svelte" but not "types" could be present)
+                (!isSvelteFilePath(moduleName) && (moduleName[0] === '.' || resolvedModule)) ||
                 // corresponding .d.ts files take precedence over .svelte files
-                tsResolvedModule?.resolvedModule?.resolvedFileName.endsWith('.d.ts') ||
-                tsResolvedModule?.resolvedModule?.resolvedFileName.endsWith('.d.svelte.ts')
+                resolvedModule?.resolvedFileName.endsWith('.d.ts') ||
+                resolvedModule?.resolvedFileName.endsWith('.d.svelte.ts')
             ) {
                 return tsResolvedModule;
             }
 
             const result = resolveSvelteModuleNameFromCache(moduleName, containingFile, options);
             // .svelte takes precedence over .svelte.ts etc
-            return result ?? tsResolvedModule;
+            return result.resolvedModule ? result : tsResolvedModule;
         });
     }
 
@@ -247,13 +260,29 @@ export function patchModuleLoader(
         options: ts.CompilerOptions
     ) {
         const cachedModule = moduleCache.get(moduleName, containingFile);
-        if (cachedModule) {
+        if (typeof cachedModule === 'object') {
             return {
-                resolvedModule: cachedModule
+                resolvedModule: cachedModule ?? undefined
             };
         }
 
         const resolvedModule = resolveSvelteModuleName(moduleName, containingFile, options);
+
+        // 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 && // TODO how to check this is not from a non-exports map?
+            // TODO check what happens if this resolves to a real .d.svelte.ts file
+            resolvedModule.extension === '.ts' // this tells us it's from an exports map
+        ) {
+            moduleCache.set(moduleName, containingFile, undefined);
+            return {
+                resolvedModule: undefined
+            };
+        }
+
         moduleCache.set(moduleName, containingFile, resolvedModule);
         return {
             resolvedModule: resolvedModule