bug#43401: 28.0.50; [PATCH] Emacs Lisp type error when the buffer passed to diff-buffer-with-file is not visiting a file

[Daniel Martín]

Steps to reproduce the problem:

emacs -Q
C-x b "Sample" RET (that is, create an empty buffer)
M-x diff-buffer-with-file RET
RET (confirm the default buffer)

Actual result:

Emacs shows "Wrong type argument: stringp, nil"

Expected result:

Emacs shows a more descriptive error that explains why the command
couldn't be executed.

I've attached a patch that implements the expected result.

Thank you very much.

0001-Check-that-the-buffer-in-diff-buffer-with-file-is-vi.patch
Description: Text Data