On Tue, Mar 10, 2026 at 09:39:05AM +0000, Jonathan Wakely wrote:
> > +#if __has_builtin(__builtin_constexpr_diag)
> > + __builtin_constexpr_diag (2, "",
> > + "std::meta::exception::via argument "
>
> s/exception/access_context/
Oops.
> > + "other than null or complete class type
> > "
> > + "reflection and exceptions disabled");
>
> I think the "and exceptions disabled" part is a bit confusing, it
> reads like it's part of the "argument other than ..." clause.
>
> You could put it in parens "... (and exceptions disabled)" or maybe
> just leave that part off completely? Users probably know when they're
> compiling with -fno-exceptions and so shouldn't be surprised they
> don't get an exception here.
The FE emits
error: exception handling disabled, use ‘-fexceptions’ to enable
for throw 1; with -fno-exceptions and
+ error_at (loc, "%qD should throw %qs; %<what()%>: %qs",
+ from, "std::meta::exception", _(what));
+ inform (loc, "exceptions are disabled, treating as non-constant; "
+ "use %qs to enable", "-fexceptions");
in the proposed metafn error patch with -fno-exceptions yesterday, so I
wanted to mention it too.
But no big deal, so removed that part now.
Here is an updated patch, tested on x86_64-linux (on reflect/*).
2026-03-10 Jakub Jelinek <[email protected]>
* 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::access_context::via argument "
+ "other than null or complete class type "
+ "reflection");
+#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::access_context::via argument
other than null or complete class type reflection" "" { target *-*-* } 0 }
Jakub
