On Sun, 3 Aug 2025, Jason Merrill wrote:
> On 8/1/25 11:16 AM, Patrick Palka wrote:
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
> > for trunk and 15/14 after 15.2 is released?
> >
> > -- >8 --
> >
> > r13-3299 changed our internal declaration of __dynamic_cast to reside
> > inside the abi / __cxxabiv1 namespace instead of the global namespace,
> > matching the real declaration. This inadvertently made us now attempt
> > constexpr evaluation of user-written calls to abi::__dynamic_cast since
> > cxx_dynamic_cast_fn_p now also returns true for them, but we're not
> > prepared to handle arbitrary calls to __dynamic_cast, and therefore ICE.
> >
> > This patch restores cxx_dynamic_cast_fn_p to return true only for
> > synthesized calls to __dynamic_cast, which can be distinguished by
> > DECL_ARTIFICIAL.
> >
> > PR c++/120620
> >
> > gcc/cp/ChangeLog:
> >
> > * constexpr.cc (cxx_dynamic_cast_fn_p): Return true only
> > for synthesized __dynamic_cast.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp2a/constexpr-dynamic19.C: New test.
> > ---
> > gcc/cp/constexpr.cc | 6 +++++-
> > gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic19.C | 10 ++++++++++
> > 2 files changed, 15 insertions(+), 1 deletion(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic19.C
> >
> > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > index e051a50fe16e..4cf94afb9ca5 100644
> > --- a/gcc/cp/constexpr.cc
> > +++ b/gcc/cp/constexpr.cc
> > @@ -3240,7 +3240,11 @@ cxx_dynamic_cast_fn_p (tree fndecl)
> > {
> > return (cxx_dialect >= cxx20
> > && id_equal (DECL_NAME (fndecl), "__dynamic_cast")
> > - && CP_DECL_CONTEXT (fndecl) == abi_node);
> > + && CP_DECL_CONTEXT (fndecl) == abi_node
> > + /* Only consider implementation-detail __dynamic_cast calls that
> > + correspond to an actual dynamic_cast, and ignore direct calls
> > + to abi::__dynamic_cast. */
> > + && DECL_ARTIFICIAL (fndecl));
>
> With this patch, does constexpr dynamic_cast still work after including
> cxxabi.h?
Yes AFAICT. I don't see duplicate_decls being called on __dynamic_cast
at all when including <cxxabi.h>, so presumably both declarations are
coexistiing. Maybe because the synthesized declaration is pushed with
hidden=true or because it's not declared as extern "C" like the real
declaration?
>
> Jason
>
>
