Re: Fixing MSVC's inability to detect elog(ERROR) does not return

On 02.09.25 04:57, David Rowley wrote:
> On Thu, 24 Jul 2025 at 23:03, Peter Eisentraut <peter@eisentraut.org> wrote:
>> Please review my patch at
>>
>> https://www.postgresql.org/message-id/ccb273c9-7544-4748-8638-30feba212e6e@eisentraut.org
>> https://commitfest.postgresql.org/patch/5934/
> 
> Now that we're building with C11, here's a rebased patch with the new
> pg_builtin_constant() macro.

+#if defined(HAVE__BUILTIN_CONSTANT_P)
+#define pg_builtin_constant(x) __builtin_constant_p(x)
+#define HAVE_PG_BUILTIN_CONSTANT
+#elif defined(_MSC_VER) && defined(__STDC_VERSION__) && 
__STDC_VERSION__ >= 201112L
+#define pg_builtin_constant(x) \
+   _Generic((1 ? ((void *) ((x) * (uintptr_t) 0)) : &(int) {1}), int *: 
1, void *: 0)
+#define HAVE_PG_BUILTIN_CONSTANT
+#endif

The variant using _Generic is not a full replacement for 
__builtin_constant_p(), because it only detects integer constant 
expressions.  So for example, it wouldn't work in the use in 
src/include/utils/memutils.h, which checks for constant strings.  So I 
think we need to be careful here to maintain the difference.

I think what we could do is make a separate macro for detecting integer 
constant expressions (like ICE_P() in the reddit thread) and define that 
to __builtin_constant_p if available, else using _Generic.  (We can't 
use _Generic on all platforms yet, that's a separate undertaking, but I 
think all platforms support either __builtin_constant_p or _Generic.) 
And then use that one for ereport.

It would also be nice to provide a comment with some explanation and/or 
a link and credit for the _Generic expression.

Btw., I think we should stick to the *_p() naming (for "predicate", I 
think) for compiler-intrinsic-affiliated functions/macros that report 
boolean results.

The __STDC_VERSION__ comparison can be dropped, since that is the 
minimum now required.  (But you need to keep defined(__STDC_VERSION__), 
since this won't work in C++.)