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libstdc++: Fix std::to_array for trivial-ish types [PR115522]

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Due to PR c++/85723 the std::is_trivial trait is true for types with a
deleted default constructor, so the use of std::is_trivial in
std::to_array is not sufficient to ensure the type can be trivially
default constructed then filled using memcpy.

I also forgot that a type with a deleted assignment operator can still
be trivial, so we also need to check that it's assignable because the
is_constant_evaluated() path can't use memcpy.

Replace the uses of std::is_trivial with std::is_trivially_copyable
(needed for memcpy), std::is_trivially_default_constructible (needed so
that the default construction is valid and does no work) and
std::is_copy_assignable (needed for the constant evaluation case).

libstdc++-v3/ChangeLog:

	PR libstdc++/115522
	* include/std/array (to_array): Workaround the fact that
	std::is_trivial is not sufficient to check that a type is
	trivially default constructible and assignable.
	* testsuite/23_containers/array/creation/115522.cc: New test.

(cherry picked from commit 510ce5eed69ee1bea9c2c696fe3b2301e16d1486)
This commit is contained in:
Jonathan Wakely 2024-06-18 13:27:02 +01:00
parent cff270707f
commit 21c8708ba6
2 changed files with 39 additions and 2 deletions
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8
libstdc++-v3/include/std/array
View file

@ -431,7 +431,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static_assert(is_constructible_v<_Tp, _Tp&>);
if constexpr (is_constructible_v<_Tp, _Tp&>)
{
if constexpr (is_trivial_v<_Tp>)
if constexpr (is_trivially_copyable_v<_Tp>
&& is_trivially_default_constructible_v<_Tp>
&& is_copy_assignable_v<_Tp>)
{
array<remove_cv_t<_Tp>, _Nm> __arr;
if (!__is_constant_evaluated() && _Nm != 0)
@ -460,7 +462,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static_assert(is_move_constructible_v<_Tp>);
if constexpr (is_move_constructible_v<_Tp>)
{
if constexpr (is_trivial_v<_Tp>)
if constexpr (is_trivially_copyable_v<_Tp>
&& is_trivially_default_constructible_v<_Tp>
&& is_copy_assignable_v<_Tp>)
{
array<remove_cv_t<_Tp>, _Nm> __arr;
if (!__is_constant_evaluated() && _Nm != 0)

33
libstdc++-v3/testsuite/23_containers/array/creation/115522.cc Normal file
View file

@ -0,0 +1,33 @@
// { dg-do compile { target c++20 } }
// PR libstdc++/115522 std::to_array no longer works for struct which is
// trivial but not default constructible
#include <array>
void
test_deleted_ctor()
{
struct S
{
S() = delete;
S(int) { }
};
S arr[1] = {{1}};
auto arr1 = std::to_array(arr);
auto arr2 = std::to_array(std::move(arr));
}
void
test_deleted_assignment()
{
struct S
{
void operator=(const S&) = delete;
};
S arr[1] = {};
auto a1 = std::to_array(arr);
auto a2 = std::to_array(std::move(arr));
}