> If a character sequence beginning with "((" would be parsed by the shell as an arithmetic expansion if preceded by a '$', shells which implement an extension whereby "((expression))" is evaluated as an arithmetic expression may treat the "((" as introducing as an arithmetic evaluation instead of a grouping command. A conforming application shall ensure that it separates the two leading '(' characters with white space to prevent the shell from performing an arithmetic evaluation.
Shells such as dash and ash do not support it, which are the most common test cases for POSIX scripts. It should also be noted that pre-increment itself is considered optional by POSIX, although += should work.
The update 2017 standard mentions ((..)) but not in the obvious place. It's under 'compound commands/grouping' ...
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V...
> If a character sequence beginning with "((" would be parsed by the shell as an arithmetic expansion if preceded by a '$', shells which implement an extension whereby "((expression))" is evaluated as an arithmetic expression may treat the "((" as introducing as an arithmetic evaluation instead of a grouping command. A conforming application shall ensure that it separates the two leading '(' characters with white space to prevent the shell from performing an arithmetic evaluation.