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c++: avoid ARM -Wunused-value [PR114970]

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Because of the __builtin_is_constant_evaluated, maybe_constant_init in
expand_default_init fails, so the constexpr constructor isn't folded until
cp_fold, which then calls cp_build_init_expr_for_ctor, which builds a
COMPOUND_EXPR in case the enclosing expression is relying on the ARM
behavior of returning 'this'.

As in other places, avoid -Wunused-value on artificial COMPOUND_EXPR.

	PR c++/114970

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_build_init_expr_for_ctor): Suppress warnings on
	return_this COMPOUND_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/is_constant_evaluated4.C: New test.
This commit is contained in:
Jason Merrill 2025-04-10 18:16:37 -04:00
parent 44478b69d7
commit 4acdfb71d4
2 changed files with 25 additions and 2 deletions
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7
gcc/cp/cp-gimplify.cc
View file

@ -1199,8 +1199,11 @@ cp_build_init_expr_for_ctor (tree call, tree init)
tree s = build_fold_indirect_ref_loc (loc, a);
init = cp_build_init_expr (s, init);
if (return_this)
init = build2_loc (loc, COMPOUND_EXPR, TREE_TYPE (call), init,
fold_convert_loc (loc, TREE_TYPE (call), a));
{
init = build2_loc (loc, COMPOUND_EXPR, TREE_TYPE (call), init,
fold_convert_loc (loc, TREE_TYPE (call), a));
suppress_warning (init);
}
return init;
}

20
gcc/testsuite/g++.dg/opt/is_constant_evaluated4.C Normal file
View file

@ -0,0 +1,20 @@
// PR c++/114970
// { dg-do compile { target c++17 } }
// { dg-additional-options "-O -Wunused-value" }
struct sv
{
const char* str;
unsigned len;
constexpr sv(const char *p): str(p), len(0)
{
if (__builtin_is_constant_evaluated ()) { len = 42; }
}
};
int main()
{
sv s ("foo");
return s.len;
}