Fall back to the plugin base when PostCSS has no from option

packages/@tailwindcss-postcss/src/index.test.ts

@@ -140,6 +140,22 @@ describe('processing without specifying a base path', () => {
   })
 })
 
+test('processing input without a `from` option falls back to the plugin `base`', async () => {
+  // PostCSS may invoke plugins without `result.opts.from` (some bundlers,
+  // including Turbopack, do this for certain CSS inputs). When that happens
+  // `path.dirname(path.resolve(''))` yields the *parent* of CWD, which made
+  // `@import 'tailwindcss'` resolution walk above the project root and fail
+  // with "Can't resolve 'tailwindcss' in '<parent of CWD>'". The fallback
+  // should be the plugin-level `base` (which itself defaults to CWD).
+  let processor = postcss([
+    tailwindcss({ base: `${__dirname}/fixtures/example-project`, optimize: { minify: false } }),
+  ])
+
+  let result = await processor.process(`@import 'tailwindcss'`)
+
+  expect(result.css.length).toBeGreaterThan(0)
+})
+
 describe('plugins', () => {
   test('local CJS plugin', async () => {
     let processor = postcss([

packages/@tailwindcss-postcss/src/index.ts

@@ -115,7 +115,16 @@ function tailwindcss(opts: PluginOptions = {}): AcceptedPlugin {
           }
 
           let context = getContextFromCache(postcss, inputFile, opts)
-          let inputBasePath = path.dirname(path.resolve(inputFile))
+          // When PostCSS is invoked without `from` (some bundlers, including
+          // Turbopack, do this for certain inputs), `inputFile` is `''`.
+          // `path.resolve('')` returns `process.cwd()`, so `path.dirname(...)`
+          // would fall back to the *parent* of CWD — wrong, and breaks
+          // `@import 'tailwindcss'` resolution against the project's
+          // `node_modules`. Fall back to the plugin-level `base` (which
+          // already defaults to `process.cwd()` and respects `opts.base`).
+          let inputBasePath = inputFile
+            ? path.dirname(path.resolve(inputFile))
+            : base
 
           // Whether this is the first build or not, if it is, then we can
           // optimize the build by not creating the compiler until we need it.