I can reproduce the bug in the latest master.  On a Windows machine,
and starting with emacs -Q, I do:
M-x customize-option RET tutorial-directory
and try to complete with C-M-i inside the editable-field, with different
names,
and different cases.
It doesn't work, because the completion is made case sensitive, even though
read-file-name-completion-ignore-case is t.

Also, the completion is made not only for directory names,
but also for file names.  This also happens on a GNU/Linux machine.

Since the 'file widget suffers from the first mentioned bug as well,
the attached patch tries to improve the completion for both of these
widgets.

Best regards,
Mauro.