BUG: assign altsep normalization in the editable path hook

mesonpy/_editable.py

@@ -349,7 +349,7 @@ def _rebuild(self) -> Node:
 
     def _path_hook(self, path: str) -> MesonpyPathFinder:
         if os.altsep:
-            path.replace(os.altsep, os.sep)
+            path = path.replace(os.altsep, os.sep)
         path, _, key = path.rpartition(os.sep)
         if path == __file__:
             tree = self._rebuild()

tests/test_editable.py

@@ -349,3 +349,45 @@ def test_editable_rebuild_error(package_purelib_and_platlib, tmp_path, verbose):
 def test_install_data(venv, editable_install_data, tmp_path):
     venv.pip('install', os.fspath(editable_install_data))
     venv.python('-c', 'import package')
+
+
+@pytest.mark.skipif(not os.altsep, reason='os.altsep is only set on Windows')
+@pytest.mark.skipif(FREE_THREADED_BUILD and CYTHON_VERSION < (3, 1, 0),
+                    reason='Cython version too old, no free-threaded CPython support')
+def test_path_hook_altsep_search_path(package_complex, tmp_path):
+    """The path hook must match submodule search paths that use os.altsep.
+
+    After a real editable build, importlib/pkgutil look up subpackages via
+    path hooks. On Windows those paths may use '/' (PYTHONPATH, pathlib,
+    Meson) even though __file__ uses '\\'. The hook normalizes altsep
+    before rpartition(os.sep); discarding the replace means the hook
+    never matches __file__ and subpackage discovery fails.
+    """
+    if os.altsep in _editable.__file__:
+        pytest.skip('__file__ already uses altsep; hook compares to it as-is')
+
+    project = mesonpy.Project(package_complex, tmp_path)
+    finder = _editable.MesonpyMetaFinder(
+        'complex', {'complex'}, os.fspath(tmp_path), project._build_command, True
+    )
+
+    try:
+        sys.meta_path.insert(0, finder)
+        sys.path_hooks.insert(0, finder._path_hook)
+
+        import complex
+        assert complex.__name__ == 'complex'
+
+        # Same location importlib would use, but spelled with os.altsep.
+        altsep_path = _editable.__file__.replace(os.sep, os.altsep) + os.altsep + 'complex'
+        importer = pkgutil.get_importer(altsep_path)
+        assert isinstance(importer, _editable.MesonpyPathFinder)
+
+        names = {module.name for module in pkgutil.iter_modules([altsep_path])}
+        assert 'more' in names
+    finally:
+        del sys.meta_path[0]
+        del sys.path_hooks[0]
+        for name in list(sys.modules):
+            if name == 'complex' or name.startswith('complex.'):
+                del sys.modules[name]