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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: C23 thoughts and opinions
Date: Fri, 24 May 2024 17:10:31 +0200
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On 23/05/2024 18:40, Keith Thompson wrote:
> Michael S <already5chosen@yahoo.com> writes:
> [...]
>> Removed
> [...]
>> 7) static_assert is not provided as a macro defined in <assert.h>
>> (becomes a keyword)
>> 8) thread_local is not provided as a macro defined in <threads.h>
>> (becomes a keyword)
>>
> [...]
>> 7) bad. Breaks existing code for weak reason
>> 8) bad. Breaks existing code for weak reason
> 
> In pre-C23, _Static_assert and _Thread_local are keywords, and
> static_assert and thread_local are macros that expand to those keywords.
> 
> In C23, _Static_assert, _Thread_local, static_assert, and thread_local
> are all keywords.  Code that simply uses the old ugly keywords would not
> break.
> 
> Code that does something like "#ifdef static_assert".  I suppose the
> headers could have retained the old macro definitions.
> 
>      #define static_assert static_assert
>      #define thread_local thread_local
> 

The sort of code that could theoretically break is when you have 
definitions like this:

#define STATIC_ASSERT_NAME_(line)       STATIC_ASSERT_NAME2_(line)
#define STATIC_ASSERT_NAME2_(line)      assertion_failed_at_line_##line
#define static_assert(claim, warning) \
     typedef struct { \
         char STATIC_ASSERT_NAME_(__COUNTER__) [(claim) ? 2 : -2]; \
     } STATIC_ASSERT_NAME_(__COUNTER__)

That works in any C version, until C23, almost as well as 
_static_assert.  I used this when C11 support was rare in the tools I used.

While using #define for a C keyword is undefined behaviour, in practice 
I think you'd have a hard time finding code and a compiler that used 
such a macro and which did not work just as well in C23 mode.

(I don't know if anyone is in the habit of declaring macros named 
"thread_local".)