bug#33400: 26.1; Order changed for overriding “nil” mode in dir-locals

[Neil Roberts]

There is a project called Piglit with a .dir_locals.el file¹ like this:

((nil . ((indent-tabs-mode . t)))
 (python-mode . ((indent-tabs-mode . nil))))

I think the intention is that most files in the project should use
indent-tabs-mode but Python files should not. This doesn’t seem to work
in 26.1 and the definition for the nil mode overrides the definition for
the Python mode and Emacs enables tab indentation for Python files. If I
swap the order of the two lines then the Python mode overrides the nil
mode and it works as intended. However, this appears to be a change in
26.1. Previously the opposite order gets the right behaviour.

In git master the behaviour is fixed again so that the order shown above
makes it work. I bisected the fix down to 97b7e58c4d34722e8b0. However
it doesn’t look like this is deliberately fixing the bug. It looks like
it’s due to the fact that a call like the following ends up reversing
the order of newvars as a side effect:

  (map-merge-with 'list 'func nil newvars)

That commit just makes it avoid calling map-merge-with when there is
only one file. So presumably the bug still exists in git master when
there are multiple files.

Perhaps you could argue that this isn’t really a bug and that having
multiple values for a directory local variable is just undefined
behaviour. However I think the previous behaviour was quite useful and
it would be nice to maintain it. Or maybe ideally it could even just say
that any more specific mode overrides any less specific mode.

This was discussed on StackExchange here:

https://emacs.stackexchange.com/questions/45998

- Neil

1. https://cgit.freedesktop.org/piglit/tree/.dir-locals.el

In GNU Emacs 26.1 (build 1, x86_64-redhat-linux-gnu, GTK+ Version 3.22.30)
 of 2018-06-26 built on buildhw-10.phx2.fedoraproject.org
Windowing system distributor 'Fedora Project', version 11.0.11906000
System Description:     Fedora release 28 (Twenty Eight)

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