https://gcc.gnu.org/g:0292e066d5e87e84ff00510724987501b3931518
commit r16-1115-g0292e066d5e87e84ff00510724987501b3931518
Author: Jason Merrill <ja...@redhat.com>
Date: Wed Jun 4 13:31:02 2025 -0400
c++: constexpr prvalues vs genericize [PR120502]
Here constexpr evaluation was getting confused by the result of
split_nonconstant_init, which leaves an INIT_EXPR from an empty CONSTRUCTOR
to be followed by member initialization. As a result
CONSTRUCTOR_NO_CLEARING was set for the time_zone, and
cxx_eval_store_expression didn't set it again for the initial clobber in the
basic_string constructor, so when cxx_fold_indirect_ref wants to check
whether the anonymous union active member had type non_trivial_if, we see
that we don't currently have a value for the anonymous union, try to add
one, and fail.
So let's do constexpr evaluation before split_nonconstant_init.
PR c++/120502
gcc/cp/ChangeLog:
* cp-gimplify.cc (cp_fold_r) [TARGET_EXPR]: Do constexpr evaluation
before genericize.
* constexpr.cc (cxx_eval_store_expression): Add comment.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/constexpr-prvalue2.C: New test.
Diff:
---
gcc/cp/constexpr.cc | 3 ++-
gcc/cp/cp-gimplify.cc | 21 ++++++++++++--------
gcc/testsuite/g++.dg/cpp2a/constexpr-prvalue2.C | 26 +++++++++++++++++++++++++
3 files changed, 41 insertions(+), 9 deletions(-)
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index b9fdc94b961a..c0e37b2fd35c 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -6413,7 +6413,8 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree
t,
if (TREE_CLOBBER_P (init)
&& CLOBBER_KIND (init) < CLOBBER_OBJECT_END)
- /* Only handle clobbers ending the lifetime of objects. */
+ /* Only handle clobbers ending the lifetime of objects.
+ ??? We should probably set CONSTRUCTOR_NO_CLEARING. */
return void_node;
/* First we figure out where we're storing to. */
diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index 16def885127e..0fcfa16d2c59 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -1473,6 +1473,19 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
break;
case TARGET_EXPR:
+ if (!flag_no_inline)
+ if (tree &init = TARGET_EXPR_INITIAL (stmt))
+ {
+ tree folded = maybe_constant_init (init, TARGET_EXPR_SLOT (stmt),
+ (data->flags & ff_mce_false
+ ? mce_false : mce_unknown));
+ if (folded != init && TREE_CONSTANT (folded))
+ init = folded;
+ }
+
+ /* This needs to happen between the constexpr evaluation (which wants
+ pre-generic trees) and fold (which wants the cp_genericize_init
+ transformations). */
if (data->flags & ff_genericize)
cp_genericize_target_expr (stmt_p);
@@ -1481,14 +1494,6 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
cp_walk_tree (&init, cp_fold_r, data, NULL);
cp_walk_tree (&TARGET_EXPR_CLEANUP (stmt), cp_fold_r, data, NULL);
*walk_subtrees = 0;
- if (!flag_no_inline)
- {
- tree folded = maybe_constant_init (init, TARGET_EXPR_SLOT (stmt),
- (data->flags & ff_mce_false
- ? mce_false : mce_unknown));
- if (folded != init && TREE_CONSTANT (folded))
- init = folded;
- }
/* Folding might replace e.g. a COND_EXPR with a TARGET_EXPR; in
that case, strip it in favor of this one. */
if (TREE_CODE (init) == TARGET_EXPR)
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-prvalue2.C
b/gcc/testsuite/g++.dg/cpp2a/constexpr-prvalue2.C
new file mode 100644
index 000000000000..c2dc7cd49456
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-prvalue2.C
@@ -0,0 +1,26 @@
+// PR c++/120502
+// { dg-do compile { target c++20 } }
+// { dg-additional-options -O }
+
+struct non_trivial_if {
+ constexpr non_trivial_if() {}
+};
+struct allocator : non_trivial_if {};
+struct padding {};
+struct __short {
+ [[no_unique_address]] padding p;
+};
+struct basic_string {
+ union {
+ __short s;
+ int l;
+ };
+ [[no_unique_address]] allocator a;
+ constexpr basic_string() {}
+ ~basic_string() {}
+};
+struct time_zone {
+ basic_string __abbrev;
+ long __offset;
+};
+time_zone convert_to_time_zone() { return {}; }
