On Tue, 10 Mar 2026 at 10:22, Jakub Jelinek <[email protected]> wrote:
>
> 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.
Ah yes, I see. Maybe the library could also use something similar,
i.e. follow the error with a separate note about why it's not an
exception:
__builtin_constexpr_diag (2, "",
"std::meta::access_context::via argument "
"other than null or complete class type "
"reflection");
__builtin_constexpr_diag (0, "", "use -fexceptions to turn
this into an exception");
Or "throwing meta::exception disabled by -fno-exception" or something.
But it's probably too early to settle on a perfect form of constexpr
diagnostics for now - we'll get more experience with them as people
start using this code.
> But no big deal, so removed that part now.
>
> Here is an updated patch, tested on x86_64-linux (on reflect/*).
OK, thanks.
>
> 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
>
