bug#68815: Unexpected behavior with read-file-name and functional REQUIRE-MATCH argument

[Stefan Monnier]

>> They should wrap PRED so as to pass the arg through
>> `substitute-in-file-name` (or otherwise arrange to make sure PRED is
>> called with an unquoted file name).
> Thank you for the clear instructions!  Does the attached patch do the
> right thing?

I theory, I think it's correct, yes.

Whether the rest of the code handles it correctly OTOH is a different
question.  E.g. I have the impression that currently the
`read-file-name-internal` completion table presumes the PRED argument
takes an already-unquoted file name.

Also, performance can be a concern (in many cases it makes more sense
to make the caller pass the unquoted name rather than force it to quote
the name only for PRED to unquote it).

IOW, I think we have a bit of a mess in our hands (that's what I was
meant by "good old problem" 🙁).

> If so, may it be applied to emacs-29?

Definitely not material for `emacs-29` I'm afraid.


        Stefan