fix: Support explicit and implicit tsconfig from packages

See microsoft/TypeScript#27348

src/index.ts

@@ -31,7 +31,28 @@ const resolveTsConfigFromFile = (cwd: string, filename: string) => {
 const resolveTsConfigFromExtends = (cwd: string, name: string) => {
   if (path.isAbsolute(name)) return fs.existsSync(name) ? name : null
   if (name.startsWith(".")) return findUp(name, cwd)
-  const id = req.resolve(name, { paths: [cwd] })
+  let id: string | null = null
+  try {
+    id = req.resolve(name, { paths: [cwd] })
+  } catch (error) {
+    if ((error as NodeJS.ErrnoException).code === "MODULE_NOT_FOUND") {
+      // config package did _not_ use pkg.main field
+      const pkgJsonPath = req.resolve(path.join(name, "package.json"), {
+        paths: [cwd],
+      })
+      const pkgManifest = req(pkgJsonPath) as { tsconfig?: string }
+      // use explicit pkg.tsconfig or implicit "index" {pkgroot}/tsconfig.json
+      id = req.resolve(
+        path.join(
+          name,
+          pkgManifest.tsconfig ? pkgManifest.tsconfig : "tsconfig.json",
+        ),
+        { paths: [cwd] },
+      )
+    } else {
+      throw error
+    }
+  }
   return id
 }
 

test/fixtures/extends-package-explicit/tsconfig.json

@@ -0,0 +1,3 @@
+{
+  "extends": "tsconfig-pkg-explicit"
+}

test/fixtures/extends-package-implicit/tsconfig.json

@@ -0,0 +1,3 @@
+{
+  "extends": "tsconfig-pkg-implicit"
+}

test/index.test.ts

@@ -26,6 +26,16 @@ test("extends package", () => {
   ])
 })
 
+test("extends package with explicit tsconfig", () => {
+  const loaded = loadTsConfig(fixture("extends-package-explicit"))
+  expect(loaded?.data.files).toEqual(["explicit"])
+})
+
+test("extends package with implicit tsconfig", () => {
+  const loaded = loadTsConfig(fixture("extends-package-implicit"))
+  expect(loaded?.data.files).toEqual(["implicit"])
+})
+
 test("find nearest file", () => {
   expect(loadTsConfig(fixture("find-nearest/nested/dir"))).not.toBe(null)
 })