https://gcc.gnu.org/g:fdbc5ff61b471076cc9c758fb6c30d62f7ef1c56
commit r16-2432-gfdbc5ff61b471076cc9c758fb6c30d62f7ef1c56
Author: Jason Merrill <ja...@redhat.com>
Date: Wed Jul 16 11:52:45 2025 -0400
c++: constexpr union placement new [PR121068]
The note and example in [class.union] p6 think that placement new can be
used to change the active member of a union, but we didn't support that for
array members in constant-evaluation even after implementing P1330 and
P2747.
First I tried to address this by introducing a CLOBBER_BEGIN_OBJECT for the
entire array, but that broke the resolution of LWG3436, which invokes 'new
T[1]' for an array T, and trying to clobber a multidimensional array when
the actual object is single-dimensional breaks. So I've raised that issue
with the committee. Until that is resolved, this patch takes a simpler
approach: allow initialization of an element of an array to make the array
the active member of a union.
PR c++/121068
gcc/cp/ChangeLog:
* constexpr.cc (cxx_eval_store_expression): Allow ARRAY_REFs
when activating an array member of a union.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/constexpr-union6.C: Expect x5 to work.
* g++.dg/cpp26/constexpr-new4.C: New test.
Diff:
---
gcc/cp/constexpr.cc | 13 ++++++++++++-
gcc/testsuite/g++.dg/cpp26/constexpr-new4.C | 21 +++++++++++++++++++++
gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C | 4 ++--
3 files changed, 35 insertions(+), 3 deletions(-)
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index ee06858f7153..1a77954b5acc 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -7736,13 +7736,24 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
tree t,
CONSTRUCTOR_APPEND_ELT (CONSTRUCTOR_ELTS (*valp), first, NULL_TREE);
/* Check for implicit change of active member for a union. */
+
+ /* LWG3436, CWG2675, c++/121068: The array object model is confused. For
+ now allow initializing an array element to activate the array. */
+ auto only_array_refs = [](const releasing_vec &refs)
+ {
+ for (unsigned i = 1; i < refs->length(); i += 3)
+ if (TREE_CODE ((*refs)[i]) != INTEGER_CST)
+ return false;
+ return true;
+ };
+
if (code == UNION_TYPE
&& (CONSTRUCTOR_NELTS (*valp) == 0
|| CONSTRUCTOR_ELT (*valp, 0)->index != index)
/* An INIT_EXPR of the last member in an access chain is always OK,
but still check implicit change of members earlier on; see
cpp2a/constexpr-union6.C. */
- && !(TREE_CODE (t) == INIT_EXPR && refs->is_empty ()))
+ && !(TREE_CODE (t) == INIT_EXPR && only_array_refs (refs)))
{
bool has_active_member = CONSTRUCTOR_NELTS (*valp) != 0;
tree inner = strip_array_types (reftype);
diff --git a/gcc/testsuite/g++.dg/cpp26/constexpr-new4.C
b/gcc/testsuite/g++.dg/cpp26/constexpr-new4.C
new file mode 100644
index 000000000000..12d8a465bbf9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/constexpr-new4.C
@@ -0,0 +1,21 @@
+// PR c++/121068
+// { dg-do compile { target c++26 } }
+
+constexpr void *operator new (__SIZE_TYPE__, void *p) { return p; }
+constexpr void *operator new[] (__SIZE_TYPE__, void *p) { return p; }
+
+consteval int
+foo()
+{
+ using T = int;
+ union { T arr[3]; };
+ new(arr) T[3]; // makes arr active
+ for (int i = 0; i < 3; ++i)
+ arr[i].~T();
+
+ new (arr + 2) T{10}; // A
+
+ return 1;
+};
+
+constexpr int g = foo();
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
b/gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
index 00bda531e596..ab8c979dc96c 100644
--- a/gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
@@ -45,9 +45,9 @@ constexpr int test5() {
union {
int data[1];
} u;
- std::construct_at(u.data, 0); // { dg-message "in .constexpr. expansion" }
+ std::construct_at(u.data, 0); // { dg-bogus "in .constexpr. expansion" }
return 0;
}
-constexpr int x5 = test5(); // { dg-message "in .constexpr. expansion" }
+constexpr int x5 = test5(); // { dg-bogus "in .constexpr. expansion" }
// { dg-error "accessing (uninitialized member|.* member instead of)" "" {
target *-*-* } 0 }
