[Bug libstdc++/126807] New: simd: Tests fail on Darwin since their introduction.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126807

            Bug ID: 126807
           Summary: simd: Tests fail on Darwin since their introduction.
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: rejects-valid, testsuite-fail
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: iains at gcc dot gnu.org
                CC: mkretz at gcc dot gnu.org
  Target Milestone: ---
            Target: *-*-darwin*

14 simd tests introduced in r16-8243-g8be0893fd98c9a have always failed on
darwin.  Since the change to add c++29 that now gives 28 fails in libstdc++. 
See, for example:
https://gcc.gnu.org/pipermail/gcc-testresults/2026-August/884410.html

The reason is that Darwin defines math_errhandling as a function rather than a
constant.  It seems that the intent was for that case to be a substitution
failure so the __handle_fpexcept_impl fallback is chosen instead.  However,
requires-clause used to do this mentions no template parameter, so it is
non-dependent and is checked at the point of definition: there is no
substitution, so there is no substitution failure and no fallback, and a
non-constant math_errhandling is a hard error.

A possible solution would be to make the math_errhandling a template param,
thus triggering SFINAE, like below - or open to other solutions (this is not
the end of the story - fixing this exposes two other issues - one of which is a
long-term compiler-side bug).  Will cross-reference those when filed.

---- possible substitution fix....

diff --git a/libstdc++-v3/include/bits/simd_details.h
b/libstdc++-v3/include/bits/simd_details.h
index a3ec38f0dc0..46eab585eb9 100644
--- a/libstdc++-v3/include/bits/simd_details.h
+++ b/libstdc++-v3/include/bits/simd_details.h
@@ -422,12 +422,14 @@ namespace simd
   // math_errhandling may expand to an extern symbol, in which case we must
assume fp exceptions
   // need to be considered. A conforming C library must define
math_errhandling, but in case it
   // isn't defined we simply use the fallback.
+  // The macro is used as default template argument (rather than in a
requires-clause) so that a
+  // non-constant math_errhandling is a substitution failure in the immediate
context instead of a
+  // hard error at the point of definition.
 #ifdef math_errhandling
-  template <int = 0>
-    requires requires { typename bool_constant<0 != (math_errhandling &
MATH_ERREXCEPT)>; }
+  template <int _ErrHandling = math_errhandling>
     consteval bool
     __handle_fpexcept_impl(int)
-    { return 0 != (math_errhandling & MATH_ERREXCEPT); }
+    { return 0 != (_ErrHandling & MATH_ERREXCEPT); }
 #endif

   // Fallback if math_errhandling doesn't work: implement correct exception
behavior.