fix(unplugin-dts): resolve tsconfig paths without baseUrl relative to config dir

When baseUrl is absent in tsconfig.json, TypeScript 5.4+ resolves paths
relative to the tsconfig.json directory. The plugin previously defaulted
to process.cwd(), causing incorrect alias resolution.

Fixes #458

Author: qmhc

packages/unplugin-dts/src/core/runtime.ts

@@ -182,18 +182,19 @@ export class Runtime {
 
     const outDirs = normalizeOutDirs(options.outDirs, root, defaultOutDir)
 
-    const {
-      // Here we are using the default value to set the `baseUrl` to the current directory if no value exists. This is
-      // the same behavior as the TS Compiler. See TS source:
-      // https://github.com/microsoft/TypeScript/blob/3386e943215613c40f68ba0b108cda1ddb7faee1/src/compiler/utilities.ts#L6493-L6501
-      baseUrl = compilerOptions.paths ? process.cwd() : undefined,
-      paths,
-    } = compilerOptions
-
-    if (pathsToAliases && baseUrl && paths) {
+    const { baseUrl, paths } = compilerOptions
+
+    // When `paths` is used without `baseUrl` in tsconfig.json, TypeScript 5.4+
+    // resolves paths relative to the containing directory of tsconfig.json.
+    const resolvedBaseUrl = baseUrl ?? (paths && configPath ? dirname(configPath) : root)
+
+    if (pathsToAliases && resolvedBaseUrl && paths) {
       aliases.push(
         ...parseTsAliases(
-          ensureAbsolute(resolveConfigDir(baseUrl, root), configPath ? dirname(configPath) : root),
+          ensureAbsolute(
+            resolveConfigDir(resolvedBaseUrl, root),
+            configPath ? dirname(configPath) : root,
+          ),
           paths,
         ),
       )

packages/unplugin-dts/tests/runtime.spec.ts

@@ -0,0 +1,83 @@
+import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { dirname, resolve } from 'node:path'
+import { afterEach, describe, expect, it } from 'vitest'
+
+import { Runtime } from '../src/core/runtime'
+import { normalizePath } from '../src/core/utils'
+
+describe('runtime tests', () => {
+  let tempDir: string
+
+  afterEach(() => {
+    if (tempDir) {
+      rmSync(tempDir, { recursive: true, force: true })
+    }
+  })
+
+  it('should resolve paths relative to tsconfig dir when baseUrl is absent', async () => {
+    tempDir = mkdtempSync(resolve(tmpdir(), 'unplugin-dts-'))
+
+    writeFileSync(
+      resolve(tempDir, 'tsconfig.json'),
+      JSON.stringify({
+        compilerOptions: {
+          paths: {
+            '@/*': ['./src/*'],
+          },
+        },
+        include: ['src/**/*'],
+      }),
+    )
+
+    mkdirSync(resolve(tempDir, 'src'))
+    writeFileSync(resolve(tempDir, 'src', 'index.ts'), 'export const foo = 1\n')
+    writeFileSync(resolve(tempDir, 'src', 'helper.ts'), 'export const bar = 2\n')
+
+    const runtime = await Runtime.toInstance({
+      root: tempDir,
+      tsconfigPath: 'tsconfig.json',
+      pathsToAliases: true,
+    })
+
+    const alias = runtime.aliases.find((a: any) =>
+      typeof a.find === 'string' ? a.find === '@/' : a.find.test('@/helper'),
+    )
+
+    expect(alias).toBeDefined()
+    expect(normalizePath(alias!.replacement)).toBe(normalizePath(resolve(tempDir, 'src/$1')))
+  })
+
+  it('should use baseUrl when explicitly set', async () => {
+    tempDir = mkdtempSync(resolve(tmpdir(), 'unplugin-dts-'))
+
+    writeFileSync(
+      resolve(tempDir, 'tsconfig.json'),
+      JSON.stringify({
+        compilerOptions: {
+          baseUrl: './lib',
+          paths: {
+            '@/*': ['./src/*'],
+          },
+        },
+        include: ['lib/src/**/*'],
+      }),
+    )
+
+    mkdirSync(resolve(tempDir, 'lib', 'src'), { recursive: true })
+    writeFileSync(resolve(tempDir, 'lib', 'src', 'index.ts'), 'export const foo = 1\n')
+
+    const runtime = await Runtime.toInstance({
+      root: tempDir,
+      tsconfigPath: 'tsconfig.json',
+      pathsToAliases: true,
+    })
+
+    const alias = runtime.aliases.find((a: any) =>
+      typeof a.find === 'string' ? a.find === '@/' : a.find.test('@/helper'),
+    )
+
+    expect(alias).toBeDefined()
+    expect(normalizePath(alias!.replacement)).toBe(normalizePath(resolve(tempDir, 'lib/src/$1')))
+  })
+})