fix(watcher): derive fallback file patterns

test/integration/watcher-typescript.test.ts

@@ -32,7 +32,7 @@ interface DiagResult {
   [serverId: string]: { diagnostics?: { message: string }[] };
 }
 
-describe("watcher: typescript handles deleted opened files", { skip: skip ?? undefined }, () => {
+describe("watcher: typescript handles derived file patterns", { skip: skip ?? undefined }, () => {
   let tmpDir: string;
   let mainFile: string;
   let helperFile: string;
@@ -81,6 +81,35 @@ describe("watcher: typescript handles deleted opened files", { skip: skip ?? und
     fs.rmSync(tmpDir, { recursive: true, force: true });
   });
 
+  it("clears missing module after an unopened imported file is created", async () => {
+    fs.rmSync(helperFile);
+
+    const missing = (await runCliJson(
+      [mainFile, "diagnostics", '{"timeoutMs":5000}'],
+      env,
+    )) as DiagResult;
+    assert.ok(
+      (missing["typescript-language-server"]?.diagnostics ?? []).some((d) =>
+        d.message.includes("Cannot find module './helper'")
+      ),
+    );
+
+    fs.writeFileSync(helperFile, "export function helper(): number {\n  return 1;\n}\n");
+
+    const result = await pollUntil(
+      async () =>
+        (await runCliJson(
+          [mainFile, "diagnostics", '{"timeoutMs":3000}'],
+          env,
+        )) as DiagResult,
+      (r) => (r["typescript-language-server"]?.diagnostics ?? []).length === 0,
+      15000,
+      500,
+    );
+
+    assert.deepEqual(result["typescript-language-server"]?.diagnostics ?? [], []);
+  });
+
   it("reports missing module after an opened imported file is deleted", async () => {
     const initial = (await runCliJson(
       [mainFile, "diagnostics", '{"timeoutMs":5000}'],