Created attachment 56884 [details]
Gzipped preprocessed source

Compile the attached .ii with '-std=gnu++20 -O' (or any non-zero optimization level) to reproduce the ICE.

Reduced to:


template <typename> class variant;

template <typename _Types, typename _Tp>
auto __variant_cast(_Tp __rhs) { return static_cast<variant<_Types>&&>(__rhs); }

template <typename _Types>
struct _Move_assign_base : _Types {
  void operator=(_Move_assign_base __rhs) { __variant_cast<_Types>(__rhs); }
};

template <typename _Types>
struct variant : _Move_assign_base<_Types> {
  void emplace() {
    variant __tmp;
    *this = __tmp;
  }
};

struct _Undefined_class {
  struct _Nocopy_types {
    void (_Undefined_class::*_M_member_pointer)();
  };
  struct function : _Nocopy_types {
    struct optional {
      void test03() {
        variant<function> v;
        v.emplace();
      }
    };
  };
};


The following patch seems to fix the immediate problem (but not yet fully regtested):

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index e1b2d27fc36..051f73fb73f 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -5709,7 +5709,8 @@ cxx_fold_indirect_ref_1 (const constexpr_ctx *ctx, location_t loc, tree type,
          }

       /* Handle conversion to "as base" type.  */
-      if (CLASSTYPE_AS_BASE (optype) == type)
+      if (CLASS_TYPE_P (optype)
+         && CLASSTYPE_AS_BASE (optype) == type)
        return op;

       /* Handle conversion to an empty base class, which is represented with a

That was quick, thanks!

The master branch has been updated by Nathaniel Shead <nshead@gcc.gnu.org>:

https://gcc.gnu.org/g:39f9c426f58448d6df340cdccd84e05721a20921

commit r14-6619-g39f9c426f58448d6df340cdccd84e05721a20921
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Sat Dec 16 10:59:03 2023 +1100

    c++: Fix unchecked use of CLASSTYPE_AS_BASE [PR113031]
    
    My previous commit (naively) assumed that a TREE_CODE of RECORD_TYPE or
    UNION_TYPE was sufficient for optype to be considered a "class type".
    However, this does not account for e.g. template type parameters of
    record or union type. This patch corrects to check for CLASS_TYPE_P
    before checking for as-base conversion.
    
            PR c++/113031
    
    gcc/cp/ChangeLog:
    
            * constexpr.cc (cxx_fold_indirect_ref_1): Check for CLASS_TYPE
            before using CLASSTYPE_AS_BASE.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp0x/pr113031.C: New test.
    
    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>

So fixed?

Yes, fixed as far as I'm aware.

.