PhpStorage\FileStorage tries to write a file using the same name of MTimeProtectedFileStorage's directory name

5: drupal8.phpstorage-directory-name.5.patch queued for re-testing.

+++ b/core/lib/Drupal/Component/PhpStorage/MTimeProtectedFastFileStorage.php
@@ -152,6 +152,13 @@ protected function getFullPath($name, &$directory = NULL, &$directory_mtime = NU
    * Returns the full path of the containing directory where the file is or should be stored.
    */
   protected function getContainingDirectoryFullPath($name) {
+    // Remove the .php file extension from the directory name. At least on
+    // Windows, a directory cannot have the same name as a file in the same
+    // folder, so when switching between MTimeProtectedFastFileStorage and
+    // FileStorage, the subdirectory cannot be created.
+    if (substr($name, -4) === '.php') {
+      $name = substr($name, 0, -4);
+    }
     return $this->directory . '/' . str_replace('/', '#', $name);

We can safely leave out the reference to Windows, other OSes behave the same.

Instead of special-casing files with php extension, wouldn't it make sense to simply substitute the periods (we already do the same with slashes). E.g:

return $this->directory . '/' . strtr($name, array('/' => '#', '.' => '^'));

I'd prefer to keep the removal of the .php extension

Ok, I'm fine with that. In fact the documentation on PhpStorageInterface suggests that file names are expected to have a .php suffix.

I've reproduced the failure condition and verified the fix. This is good to go.

Committed 241356f and pushed to 8.x. Thanks!