https://gcc.gnu.org/g:f23b5df56e237df9f66b615ca4babc564d5f75de
commit r16-2265-gf23b5df56e237df9f66b615ca4babc564d5f75de
Author: Jason Merrill <ja...@redhat.com>
Date: Sat Jul 12 05:15:01 2025 -0400
c++: constexpr uninitialized union [PR120577]
This was failing for two reasons:
1) We were wrongly treating the basic_string constructor as
zero-initializing the object, which it doesn't.
2) Given that, when we went to look for a value for the anonymous union,
we concluded that it was value-initialized, and trying to evaluate that
broke because we weren't setting ctx->ctor for it.
This patch fixes both issues, #1 by setting CONSTRUCTOR_NO_CLEARING and #2
by inserting a new CONSTRUCTOR for the member rather than evaluate it out of
context, which is consistent with cxx_eval_store_expression.
PR c++/120577
gcc/cp/ChangeLog:
* constexpr.cc (cxx_eval_call_expression): Set
CONSTRUCTOR_NO_CLEARING on initial value for ctor.
(cxx_eval_component_reference): Make value-initialization
of an aggregate member explicit.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/constexpr-union9.C: New test.
Diff:
---
gcc/cp/constexpr.cc | 18 ++++++++++++---
gcc/testsuite/g++.dg/cpp2a/constexpr-union9.C | 33 +++++++++++++++++++++++++++
2 files changed, 48 insertions(+), 3 deletions(-)
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index eb19784dbbaa..ee06858f7153 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -4201,10 +4201,10 @@ cxx_eval_call_expression (const constexpr_ctx *ctx,
tree t,
&& TREE_CODE (new_obj) == COMPONENT_REF
&& TREE_CODE (TREE_TYPE (TREE_OPERAND (new_obj, 0))) ==
UNION_TYPE)
{
+ tree ctor = build_constructor (TREE_TYPE (new_obj), NULL);
+ CONSTRUCTOR_NO_CLEARING (ctor) = true;
tree activate = build2 (INIT_EXPR, TREE_TYPE (new_obj),
- new_obj,
- build_constructor (TREE_TYPE (new_obj),
- NULL));
+ new_obj, ctor);
cxx_eval_constant_expression (ctx, activate,
lval, non_constant_p, overflow_p,
jump_target);
@@ -5793,6 +5793,18 @@ cxx_eval_component_reference (const constexpr_ctx *ctx,
tree t,
}
/* If there's no explicit init for this field, it's value-initialized. */
+
+ if (AGGREGATE_TYPE_P (TREE_TYPE (t)))
+ {
+ /* As in cxx_eval_store_expression, insert an empty CONSTRUCTOR
+ and copy the flags. */
+ constructor_elt *e = get_or_insert_ctor_field (whole, part);
+ e->value = value = build_constructor (TREE_TYPE (part), NULL);
+ CONSTRUCTOR_ZERO_PADDING_BITS (value)
+ = CONSTRUCTOR_ZERO_PADDING_BITS (whole);
+ return value;
+ }
+
value = build_value_init (TREE_TYPE (t), tf_warning_or_error);
return cxx_eval_constant_expression (ctx, value,
lval,
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union9.C
b/gcc/testsuite/g++.dg/cpp2a/constexpr-union9.C
new file mode 100644
index 000000000000..7db1030ab200
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union9.C
@@ -0,0 +1,33 @@
+// PR c++/120577
+// { dg-do compile { target c++20 } }
+
+template <class _Tp> struct optional {
+ union {
+ _Tp __val_;
+ };
+ template <class... _Args>
+ constexpr optional(_Args... __args)
+ : __val_(__args...) {}
+};
+template <class _Tp, class... _Args>
+constexpr optional<_Tp> make_optional(_Args... __args) {
+ return optional<_Tp>(__args...);
+}
+
+struct __non_trivial_if {
+ constexpr __non_trivial_if() {}
+};
+struct allocator : __non_trivial_if {};
+struct __padding {};
+struct __short {
+ [[__no_unique_address__]] __padding __padding_;
+ int __data_;
+};
+struct basic_string {
+ union {
+ __short __s;
+ };
+ [[__no_unique_address__]] allocator __alloc_;
+ constexpr basic_string(int, int) {}
+};
+auto opt = make_optional<basic_string>(4, 'X');
