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c++: empty union member activation during constexpr [PR102163]

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Here, the union's constructor is defined to activate its empty data
member _M_rest, but during constexpr evaluation of this constructor the
subobject constructor call O::O(&_M_rest, 42) doesn't produce a side
effect that actually activates the member, so the union still appears
uninitialized after its constructor has run.  This patch fixes this by
using a dummy MODIFY_EXPR in this situation, whose evaluation ensures
the member gets activated.

	PR c++/102163

gcc/cp/ChangeLog:

	* constexpr.c (cxx_eval_call_expression): After evaluating a
	subobject constructor call for an empty union member, produce a
	side effect that makes sure the member gets activated.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-empty17.C: New test.
This commit is contained in:
Patrick Palka 2021-09-14 11:22:12 -04:00
parent 818c505188
commit de07cff96a
2 changed files with 49 additions and 6 deletions
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34
gcc/cp/constexpr.c
View file

@ -2787,12 +2787,34 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
&jump_target);
if (DECL_CONSTRUCTOR_P (fun))
/* This can be null for a subobject constructor call, in
which case what we care about is the initialization
side-effects rather than the value. We could get at the
value by evaluating *this, but we don't bother; there's
no need to put such a call in the hash table. */
result = lval ? ctx->object : ctx->ctor;
{
/* This can be null for a subobject constructor call, in
which case what we care about is the initialization
side-effects rather than the value. We could get at the
value by evaluating *this, but we don't bother; there's
no need to put such a call in the hash table. */
result = lval ? ctx->object : ctx->ctor;
/* If we've just evaluated a subobject constructor call for an
empty union member, it might not have produced a side effect
that actually activated the union member. So produce such a
side effect now to ensure the union appears initialized. */
if (!result && new_obj
&& TREE_CODE (new_obj) == COMPONENT_REF
&& TREE_CODE (TREE_TYPE
(TREE_OPERAND (new_obj, 0))) == UNION_TYPE
&& is_really_empty_class (TREE_TYPE (new_obj),
/*ignore_vptr*/false))
{
tree activate = build2 (MODIFY_EXPR, TREE_TYPE (new_obj),
new_obj,
build_constructor (TREE_TYPE (new_obj),
NULL));
cxx_eval_constant_expression (ctx, activate, lval,
non_constant_p, overflow_p);
ggc_free (activate);
}
}
else if (VOID_TYPE_P (TREE_TYPE (res)))
result = void_node;
else

21
gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C Normal file
View file

@ -0,0 +1,21 @@
// PR c++/102163
// { dg-do compile { target c++11 } }
struct O {
constexpr O(int) { }
};
union _Variadic_union {
constexpr _Variadic_union(int __arg) : _M_rest(__arg) { }
int _M_first;
O _M_rest;
};
constexpr _Variadic_union u(42);
struct _Variant_storage {
constexpr _Variant_storage() : _M_u(42) {}
_Variadic_union _M_u;
};
constexpr _Variant_storage w;