On 8/3/25 12:23 PM, Patrick Palka wrote:
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?
Then please add a testcase for that case or add #include <cxxabi.h> to
an existing constexpr dynamic_cast test. OK with that addition.
Jason
