[PATCH] libstdc++: Use __builtin_constexpr_diag instead of __asm__("") in <meta> if possible

[Jakub Jelinek]

Hi!

Your asm -> __asm__ patch made me think that these are perfect
candidates for __builtin_constexpr_diag, the asms were there just
to make it provably non-constant expression (guess *(char*)nullptr = 0;
would do as well).  But with __builtin_constexpr_diag we can tell
the user the details.

Additionally, when testing it I've found that the
https://eel.is/c++draft/meta.reflection#access.context-13
Throws: meta::exception unless cls is either the null reflection
or a reflection of a complete class type.
part of via's description was implemented wierdly, it did throw
an exception, but telling user that via has been called with
something that is not a null reflection nor reflection of a complete
class type, but that is_class_type has been called on a reflection
of something other than a type.

Tested on x86_64-linux, ok for trunk?

2026-03-10  Jakub Jelinek  <jakub@redhat.com>

	* include/std/meta (std::meta::exception::what()): Use
	__builtin_constexpr_diag instead of __asm__("") if supported.
	(std::meta::access_context::via(info)): Don't call is_class_type
	if __cls is not a type.  Use __builtin_constexpr_diag instead of
	__asm__("") if supported for -fno-exceptions.

	* g++.dg/reflect/eh8.C: Expect different diagnostics.
	* g++.dg/reflect/no-exceptions2.C: New test.

--- libstdc++-v3/include/std/meta.jj	2026-03-10 08:14:28.100322627 +0100
+++ libstdc++-v3/include/std/meta	2026-03-10 10:03:33.295442677 +0100
@@ -120,7 +120,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	// from UTF-8 to ordinary literal encoding failed.
 	// In that case what() should be non-constant.
 	if (_M_what.size() == 0 && _M_u8what.size() != 0)
+#if __has_builtin(__builtin_constexpr_diag)
+	  __builtin_constexpr_diag (2, "",
+				    "std::meta::exception message could not "
+				    "be successfully transcoded from UTF-8 to "
+				    "ordinary literal encoding");
+#else
 	  __asm__("");
+#endif
 	return _M_what.c_str();
       }
       consteval u8string_view u8what() const noexcept { return _M_u8what; }
@@ -591,7 +598,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     access_context::via(info __cls) const
     {
       if (__cls != info {}
-	  && (!std::meta::is_class_type(__cls)
+	  && (!std::meta::is_type(__cls)
+	      || !std::meta::is_class_type(__cls)
 	      || !std::meta::is_complete_type(__cls)))
 	{
 #if __cpp_exceptions
@@ -599,7 +607,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 				       "or complete class type reflection",
 				     ^^access_context::via);
 #else
+#if __has_builtin(__builtin_constexpr_diag)
+	  __builtin_constexpr_diag (2, "",
+				    "std::meta::exception::via argument "
+				    "other than null or complete class type "
+				    "reflection and exceptions disabled");
+#else
 	  __asm__("");
+#endif
 	  return *this;
 #endif
 	}
--- gcc/testsuite/g++.dg/reflect/eh8.C.jj	2026-01-15 16:33:47.007097959 +0100
+++ gcc/testsuite/g++.dg/reflect/eh8.C	2026-03-10 09:57:15.887817046 +0100
@@ -24,7 +24,7 @@ bar ()
 {
   exception a (u8"\N{GRINNING FACE}\N{GRINNING FACE WITH SMILING EYES}\N{LEFT SPEECH BUBBLE}", ^^foo);
   const char *b = a.what ();	// { dg-message "in 'constexpr' expansion of 'a.std::meta::exception::what\\\(\\\)" }
-  return true;			// { dg-error "inline assembly is not a constant expression" "" { target *-*-* } 0 }
+  return true;			// { dg-error "constexpr message: std::meta::exception message could not be successfully transcoded from UTF-8 to ordinary literal encoding" "" { target *-*-* } 0 }
 }
 
 static_assert (foo ());
--- gcc/testsuite/g++.dg/reflect/no-exceptions2.C.jj	2026-03-10 09:49:10.810013159 +0100
+++ gcc/testsuite/g++.dg/reflect/no-exceptions2.C	2026-03-10 10:07:44.471200349 +0100
@@ -0,0 +1,15 @@
+// { dg-do compile { target c++26 } }
+// { dg-additional-options "-freflection -fno-exceptions" }
+
+#include <meta>
+
+consteval bool
+foo ()
+{
+  auto ctx = std::meta::access_context::unchecked ();
+  auto ctx2 = ctx.via (^^::);	// { dg-message "in 'constexpr' expansion of 'ctx.std::meta::access_context::via\\\(\\\^\\\^::\\\)'" }
+  return true;
+}
+
+auto b = foo ();		// { dg-message "in 'constexpr' expansion of 'foo\\\(\\\)'" }
+// { dg-error "constexpr message: std::meta::exception::via argument other than null or complete class type reflection and exceptions disabled" "" { target *-*-* } 0 }

	Jakub