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Suboptimal bug fix in CC Mode.

From: Alan Mackenzie
Subject: Suboptimal bug fix in CC Mode.
Date: Mon, 15 Jan 2018 22:00:12 +0000
User-agent: Mutt/1.7.2 (2016-11-26)

Hello, Eli.

Unfortunately, a sub-optimal bug fix with unintended consequences has
found its way into Emacs-26 (and master).  The bug fix was a new CC Mode
feature, C99 compound literals.  This involves allowing brace blocks in
more contexts than previously, where a brace block essentially only
followed "=".

The "fix" adopted was to examine the inside of "{ ... }" to determine
whether it's a brace block or a statement/defun block.  Essentially, a
"," means brace block and ";" means statement/defun block.

This strategem falls down in the following use case.  Type in

    int main () {
        int a

, where the closing "}" is optional.  Now type a "," after the
identifier "a".  CC Mode now interprets the line as a brace-block-entry,
and indents it suboptimally.  This indentation can need fixing by hand,
after a ";" has caused the block to become a defun block.

This new feature, compound literals, was requested by Nikolai Weibull in
emacs-devel on 2017-08-15, and the "fix" committed by me on 2017-11-10.
I became aware of the defects of this "fix" two days ago, when bug
#29959 was reported by Konstantin Kharlamov.

I don't think we should leave this bug in Emacs-26.

Clearly, analysing the innards of "{ ... }" is not a reliable way to
distinguish brace blocks from statement/defun blocks.  Instead, more
elaborate analysis of its context needs to be carried out.  This fix
will be quite involved, and will be considerably more extensive than a
"one-line fix".

Alternatively, the C99 compound literals feature could be removed from
Emacs-26 and postponed till Emacs-27.


Alan Mackenzie (Nuremberg, Germany).

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