bug#40949: 26.3; substitute-env-in-file-name: Undefined variables not substituted

[Lars Ingebrigtsen]

Marcus Harnisch <marcus.harnisch@gmx.net> writes:

>> Whether Emacs should follow what the shell does is a separate issue.
>> In this case, since we use this function in substitute-in-file-name,
>> what it does should serve what substitute-in-file-name was always
>> doing: it left the $foo constructs unaltered.
> It is probably moot to argue against long-standing practice, but the
> behaviour could perhaps benefit from a note in docstrings of functions
> that rely on `s-e-i-f-n'.

Makes sense -- it is somewhat surprising behaviour, so I've now done
this in Emacs 28 for substitute-in-file-name.

-- 
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