[gcc r16-6954] c++: Fix ICE with constant evaluation of a = {CLOBBER} with ptrmemfn [PR123677]

[Jakub Jelinek]

https://gcc.gnu.org/g:9abe0b4f76265a062f1646a0bff1f58b70ae5695

commit r16-6954-g9abe0b4f76265a062f1646a0bff1f58b70ae5695
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Wed Jan 21 14:30:03 2026 +0100

    c++: Fix ICE with constant evaluation of a = {CLOBBER} with ptrmemfn [PR123677]
    
    The following testcase ICEs when we evaluate a = {CLOBBER} stmt.
    The code assumes that if type is an aggregate type and *valp is
    non-NULL, then it must be a CONSTRUCTOR.
    That is usually the case, but there is one exception, *valp can
    be a PTRMEM_CST if TYPE_PTRMEMFUNC_P (type) and in that
    case CONSTRUCTOR_ELTS (*valp) obviously ICEs or misbehaves.
    
    Now, while I could do something like
      if (*valp && (!TYPE_PTRMEMFUNC_P (type) || TREE_CODE (*valp) != PTRMEM_CST))
    just making sure TREE_CODE (*valp) == CONSTRUCTOR seems much easier
    and more readable.
    
    2026-01-21  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/123677
            * constexpr.cc (cxx_eval_store_expression): Only clear
            CONSTRUCTOR_ELTS (*valp) if *valp is CONSTRUCTOR.
    
            * g++.dg/cpp2a/pr123677.C: New test.

Diff:
---
 gcc/cp/constexpr.cc                   | 2 +-
 gcc/testsuite/g++.dg/cpp2a/pr123677.C | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 0873226bc061..b850bd7ced26 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -8197,7 +8197,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
     {
       if (AGGREGATE_TYPE_P (type))
 	{
-	  if (*valp)
+	  if (*valp && TREE_CODE (*valp) == CONSTRUCTOR)
 	    CONSTRUCTOR_ELTS (*valp) = nullptr;
 	  else
 	    *valp = build_constructor (type, nullptr);
diff --git a/gcc/testsuite/g++.dg/cpp2a/pr123677.C b/gcc/testsuite/g++.dg/cpp2a/pr123677.C
new file mode 100644
index 000000000000..160f1901393c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/pr123677.C
@@ -0,0 +1,7 @@
+// PR c++/123677
+// { dg-do compile { target c++20 } }
+
+struct B { void foo (); };
+typedef void (B::*A) ();
+struct C { constexpr C (A d) { auto e = new A (d); e->~A (); } };
+C c { &B::foo };