Commit: runtime(config): mark unportable += as an error

[Christian Brabandt]

runtime(config): mark unportable += as an error

Commit: https://github.com/vim/vim/commit/1e7a288cd3e522d70ad857a0a4d0ea7797f73bae
Author: Damien Lejay <dam...@lejay.be>
Date: Sun Sep 14 10:54:33 2025 -0400

runtime(config): mark unportable += as an error

closes: https://github.com/vim/vim/issues/18292

Signed-off-by: Damien Lejay <dam...@lejay.be>
Signed-off-by: Christian Brabandt <c...@256bit.org>

diff --git a/runtime/syntax/config.vim b/runtime/syntax/config.vim
index 3636ed413..d13e89b39 100644
--- a/runtime/syntax/config.vim
+++ b/runtime/syntax/config.vim
@@ -1,10 +1,11 @@
" Vim syntax file
-" Language: configure.in script: M4 with sh
+" Language: Autoconf M4
" Former Maintainer: Christian Hammesr <c...@lathspell.westend.com>
" Last Change: 2018 Feb 03
" (patch from Yngve Inntjore Levinsen to detect AC_MSG)
" (patch from Khym Chanur to add @Spell)
" (patch from James McCoy to fix paren matching)
+" (2025 Sep 14 patch from Damien Lejay to detect unportable +=)

" Well, I actually even do not know much about m4. This explains why there
" is probably very much missing here, yet !
@@ -34,6 +35,9 @@ syn region configstring start=+\z(["'`]\)+ skip=+\\z1+ end=+\z1+ contains=@
syn region configmsg matchgroup=configfunction start="AC_MSG_[A-Z]*\ze(\[" matchgroup=configdelimiter end="\])" contains=configdelimiter,@Spell
syn region configmsg matchgroup=configfunction start="AC_MSG_[A-Z]*\ze([^[]" matchgroup=configdelimiter end=")" contains=configdelimiter,@Spell

+" Help write portable shell code
+syn match acPlusEq ' \+\=' containedin=ALLBUT,configcomment
+
" Define the default highlighting.
" Only when an item doesn't have highlighting yet

@@ -47,6 +51,7 @@ hi def link configkeyword Keyword
hi def link configspecial Special
hi def link configstring String
hi def link configmsg String
+hi def link acPlusEq Error


let b:current_syntax = "config"