Re: C2x features status

[Joseph Myers <joseph@codesourcery.com>]

On Fri, 21 Oct 2022, Florian Weimer via Gcc wrote:

> Do you have a list of C2X features that are likely to impact autoconf
> tests?  Or planned changes in the GCC 13 and 14 default language modes
> that reject constructs previous accepted as an extension?

I think by far the biggest risk - for builds of old code in general, not 
necessarily for autoconf tests - comes from the following two changes / 
groups of changes:

* Change of () in function declarations and definitions to mean (void), as 
in C++, instead of an unprototyped function.

* New keywords formerly in the user namespace: alignas alignof bool 
constexpr false nullptr static_assert thread_local true typeof 
typeof_unqual.  (I haven't yet implemented C2x constexpr, but hope to get 
to it for GCC 13.  The others are all keywords now in C2x mode on GCC 
mainline.)  Of these, by far the greatest risk is probably from bool, 
false and true, while typeof was enabled by default for -std=gnu* anyway 
in previous releases so is a lower risk.

There are also many new library functions, which obviously pose a risk to 
programs using those identifiers for their own purposes and also including 
the relevant header.  I hope the risk from those is lower in general.  
Many of those aren't yet implemented in glibc; I've been focusing on 
compiler rather than library features during GCC development stage 1, and 
also hoping to be able to wait for MPFR 4.2 to be released before 
implementing many of the new <math.h> functions (to avoid 
gen-auto-libm-tests depending on an unreleased MPFR version).

-- 
Joseph S. Myers
joseph@codesourcery.com