GetFileAttributesW() doesn't follow a reparse point, such as a symlink, in the final component of the path. If tcl_library_path is the path of a symlink, checking existence of the target requires CreateFileW(wpath, 0, 0, NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL). If the call succeeds, i.e. the call doesn't return INVALID_HANDLE_VALUE, the target exists and is accessible; close the handle.

Here's the suggested implementation:

static int
file_exists(PyObject *path)
{
    wchar_t *wpath = PyUnicode_AsWideCharString(path, NULL);
    if (wpath == NULL) {
        return -2;
    }

    HANDLE hfile = CreateFileW(wpath, 0, 0, NULL, OPEN_EXISTING,
                        FILE_FLAG_BACKUP_SEMANTICS, NULL);
    if (hfile != INVALID_HANDLE_VALUE) {
        CloseHandle(hfile);
    }
    
    PyMem_Free(wpath);

    return (hfile == INVALID_HANDLE_VALUE) ? -1 : 0;
}

This returns -1 when the file doesn't exist, since file_exists() replaces _Py_stat().

I didn't expect that checking if a file exists would be so "complicated" :-) I'm waiting for @pacampbell to see if further changes are needed. Maybe this PR is not needed to build Python with clang.

The GetFileAttributesW() call is like checking for existence with lstat() instead of stat(). That's probably good enough for tcl_library_path. Python's r"tcl\tclX.Y" is installed as a normal directory. However, I don't think the CreateFileW() call is complicated, and it makes file_exists() generally correct.

For the current implementation, the use of res == -1 in _get_tcl_lib_path() is wrong. The current implementation returns 1 if the file exists and 0 if it doesn't exist. This has to be fixed even if you keep the GetFileAttributesW() call.