fix: Support subpaths of extended packages

See microsoft/TypeScript#27348

src/index.ts

@@ -37,18 +37,30 @@ const resolveTsConfigFromExtends = (cwd: string, name: string) => {
   } 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] },
-      )
+      const nameParts = name.split("/")
+      if (
+        (name.startsWith("@") && nameParts.length === 2) ||
+        nameParts.length === 1
+      ) {
+        // a module (with optional scope) lacking subpath
+        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 {
+        // a subpath directory, all others are handled in try block
+        id = req.resolve([...nameParts, "tsconfig.json"].join("/"), {
+          paths: [cwd],
+        })
+      }
     } else {
       throw error
     }

test/fixtures/extends-package-explicit/no-extn.json

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

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

@@ -0,0 +1,3 @@
+{
+  "extends": "tsconfig-pkg-explicit/other.json"
+}

test/fixtures/extends-package-implicit/no-extn.json

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

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

@@ -0,0 +1,4 @@
+{
+  "extends": "tsconfig-pkg-implicit/other/config"
+}
+

test/index.test.ts

@@ -31,11 +31,43 @@ test("extends package with explicit tsconfig", () => {
   expect(loaded?.data.files).toEqual(["explicit"])
 })
 
+test("extends package subpath file without extension", () => {
+  const loaded = loadTsConfig(
+    fixture("extends-package-explicit"),
+    "no-extn.json",
+  )
+  expect(loaded?.data.files).toEqual(["other-file"])
+})
+
+test("extends package subpath file with extension", () => {
+  const loaded = loadTsConfig(
+    fixture("extends-package-explicit"),
+    "subpath.json",
+  )
+  expect(loaded?.data.files).toEqual(["other-file"])
+})
+
 test("extends package with implicit tsconfig", () => {
   const loaded = loadTsConfig(fixture("extends-package-implicit"))
   expect(loaded?.data.files).toEqual(["implicit"])
 })
 
+test("extends package with implicit subpath directory", () => {
+  const loaded = loadTsConfig(
+    fixture("extends-package-implicit"),
+    "no-extn.json",
+  )
+  expect(loaded?.data.files).toEqual(["other-directory"])
+})
+
+test("extends package with implicit file subpath", () => {
+  const loaded = loadTsConfig(
+    fixture("extends-package-implicit"),
+    "subpath.json",
+  )
+  expect(loaded?.data.files).toEqual(["other-directory-config"])
+})
+
 test("find nearest file", () => {
   expect(loadTsConfig(fixture("find-nearest/nested/dir"))).not.toBe(null)
 })