Add test

Author: encukou

Lib/test/test_importlib/test_util.py

@@ -580,6 +580,18 @@ def test_cache_from_source_respects_pycache_prefix_relative(self):
                 self.util.cache_from_source(path, optimization=''),
                 os.path.normpath(expect))
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
+    def test_cache_from_source_in_root_with_pycache_prefix(self):
+        # Regression test for gh-82916
+        pycache_prefix = os.path.join(os.path.sep, 'tmp', 'bytecode')
+        path = 'qux.py'
+        expect = os.path.join(os.path.sep, 'tmp', 'bytecode',
+                              f'qux.{self.tag}.pyc')
+        with util.temporary_pycache_prefix(pycache_prefix):
+            with os_helper.change_cwd('/'):
+                self.assertEqual(self.util.cache_from_source(path), expect)
+
     @unittest.skipIf(sys.implementation.cache_tag is None,
                      'requires sys.implementation.cache_tag to not be None')
     def test_source_from_cache_inside_pycache_prefix(self):