[gcc r16-5488] c++: Fix up build_data_member_initialization [PR121445]

[Jakub Jelinek]

https://gcc.gnu.org/g:0ac5a60c37d9a40e8993b8207e904b713e73b5d3

commit r16-5488-g0ac5a60c37d9a40e8993b8207e904b713e73b5d3
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Nov 21 14:17:01 2025 +0100

    c++: Fix up build_data_member_initialization [PR121445]
    
    The following testcase ICEs, because the constexpr ctor in C++14
    or later doesn't contain any member initializers and so the
    massage_constexpr_body -> build_constexpr_constructor_member_initializers
    -> build_data_member_initialization member initialization discovery
    looks at the ctor body instead.  And while it has various
    cases where it punts, including COMPONENT_REF with a VAR_DECL as first
    operand on lhs of INIT_EXPR, here there is COMPONENT_REF with
    several COMPONENT_REFs and VAR_DECL only inside the innermost.
    
    The following patch makes sure we punt on those as well, instead of
    blindly assuming it is anonymous union member initializer or asserting
    it is a vtable store.
    
    An alternative to this would be some flag on the INIT_EXPRs created
    by perform_member_init and let build_data_member_initialization inspect
    only INIT_EXPRs with that flag set.
    
    2025-11-21  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/121445
            * constexpr.cc (build_data_member_initialization): Just return
            false if member is COMPONENT_REF of COMPONENT_REF with
            VAR_P get_base_address.
    
            * g++.dg/cpp1y/constexpr-121445.C: New test.

Diff:
---
 gcc/cp/constexpr.cc                           |  3 +++
 gcc/testsuite/g++.dg/cpp1y/constexpr-121445.C | 19 +++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 016e3db927c9..e091e8643a01 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -477,6 +477,9 @@ build_data_member_initialization (tree t, vec<constructor_elt, va_gc> **vec)
       if (TREE_CODE (aggr) != COMPONENT_REF)
 	/* Normal member initialization.  */
 	member = TREE_OPERAND (member, 1);
+      else if (VAR_P (get_base_address (aggr)))
+	/* Initializing a local variable, don't add anything.  */
+	return true;
       else if (ANON_AGGR_TYPE_P (TREE_TYPE (aggr)))
 	/* Initializing a member of an anonymous union.  */
 	return build_anon_member_initialization (member, init, vec);
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-121445.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-121445.C
new file mode 100644
index 000000000000..b210aaffaf40
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-121445.C
@@ -0,0 +1,19 @@
+// PR c++/121445
+// { dg-do compile { target c++14 } }
+
+struct D {
+  const char *d;
+  const char *foo () { return ""; };			// { dg-message "declared here" "" { target c++20_down } }
+  constexpr D () : d (foo ()) {}			// { dg-error "call to non-'constexpr' function 'const char\\\* D::foo\\\(\\\)'" "" { target c++20_down } }
+  constexpr D &operator= (const char *) { return *this; }
+};
+struct S {
+  constexpr S ()
+  {
+    struct A { D a; };
+    struct B { A b; };
+    struct C { B c; };
+    C d {};
+    d.c.b.a = "";
+  }
+};