[PATCH] libstdc++: Fix constexpr exceptions for -fno-exceptions

[Jonathan Wakely]

On Fri, 11 Jul 2025 at 14:02, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> The if-consteval branches in std::make_exception_ptr and
> std::exception_ptr_cast use a try-catch block, which gives an error for
> -fno-exceptions. Just make them return a null pointer at compile-time
> when -fno-exceptions is used, because there's no way to get an active
> exception with -fno-exceptions.
>
> For std::exception_ptr_cast the consteval branch doesn't depend on RTTI
> being enabled, so we can move the check for __cpp_rtti into the runtime
> branch. We can also remove the #else group and just fall through to the
> return nullptr statement if there was no return from whichever branch of
> the if-consteval was taken.

Actually for exception_ptr_cast maybe we want:

#ifdef __cpp_rtti
      if not consteval {
    const type_info &__id = typeid(const _Ex&);
    return static_cast<const _Ex*>(__p._M_exception_ptr_cast(__id));
      }
#endif

#ifdef __cpp_exceptions
      if (__p._M_exception_object)
        try
      {
        std::rethrow_exception(__p);
      }
      catch (const _Ex& __exc)
      {
        return &__exc;
      }
      catch (...)
      {
      }
#endif
      return nullptr;

So for runtime calls with -frtti we try to use that, and then unless
-fno-exceptions is used we just use try-catch.

That will allow it to work at runtime with either -fno-rtti or
-fno-exceptions (but not both).

For compile-time -fno-rtti doesn't matter, but -fno-exceptions makes
it return null.