bug#50538: [PATCH] 28.0.50; electric-pair-mode fails to pair double quotes in some cases in CC mode

[Jim Porter]

On 9/11/2021 11:26 PM, Eli Zaretskii wrote:
> > From: Jim Porter <jporterbugs@gmail.com>
> > Date: Sat, 11 Sep 2021 20:58:47 -0700
> >
> > There are a few related issues with pairing double quotes in CC mode
> > while using `electric-pair-mode'. Hopefully the steps to reproduce below
> > will explain the issues. In all the cases, I'd expect
> > `electric-pair-mode' to insert a closing quote, but it doesn't.
>
> Your expected results seem to expect Emacs to assume that a new string
> will be inserted, but is that an assumption that is always true?

In these cases, I believe that's true (with the default 
`electric-pair-mode' settings, that is). More broadly, the goal of the 
patch is to ensure that pairing of double quotes works the same in CC 
mode as it does in other modes (`ruby-mode', `python-mode', `js-mode', 
`emacs-lisp-mode', etc), which is why I added `c-mode' to the list of 
modes to test in `test/lisp/electric-tests.el'.

That said, there's one potential case I didn't account for (mostly 
because it wasn't accounted for in the patch for bug#36474): if a user 
customizes `electric-pair-inhibit-predicate' to inhibit cases like (2) 
or (3) in my original message, that won't work right in CC modes, since 
the default value of `electric-pair-inhibit-predicate' (set by the user) 
won't be called.

Attached is an updated patch that changes the logic of 
`c-electric-pair-inhibit-predicate' to either a) inhibit insertion of 
the closing quote, or b) call the default-value of 
`electric-pair-inhibit-predicate' to determine what to do. This should 
give users more consistent behavior when customizing 
`electric-pair-inhibit-predicate'.

The tests still pass, although I wasn't able to figure out a good way to 
add a test for setting `electric-pair-inhibit-predicate' that works with 
how CC mode overrides it (using `:bindings' in 
`define-electric-pair-test' didn't work, since the binding is set too 
late). Hopefully that's ok; if not, I can try and see about making some 
more extensive changes to the tests to account for this.

Note however that this solution isn't perfect: it means a user's custom 
`electric-pair-inhibit-predicate' can only inhibit *more* than CC mode's 
default behavior, not less. I think that's a reasonable compromise 
though, and users who want more direct control can set 
`electric-pair-inhibit-predicate' inside `c-mode-common-hook'. A 
"perfect" solution here would probably require adding new customization 
points to `electric-pair-mode' (e.g. a way for major modes to override 
how the syntax is analyzed), and I'm not sure the added complexity would 
be worth it, especially since this code is already a bit tricky.

0001-Improve-behavior-of-electric-pair-mode-in-cc-mode.patch
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