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[libc++] Re-implement LWG2770 again * 2 #132598

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Mar 25, 2025
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[libc++] Re-implement LWG2770 again * 2 #132598

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1 change: 1 addition & 0 deletions libcxx/docs/Status/Cxx17Issues.csv
View file
Original file line number Diff line number Diff line change
@@ -252,6 +252,7 @@
"`LWG2760 <https://wg21.link/LWG2760>`__","non-const basic_string::data should not invalidate iterators","2016-11 (Issaquah)","|Complete|","",""
"`LWG2765 <https://wg21.link/LWG2765>`__","Did LWG 1123 go too far?","2016-11 (Issaquah)","|Complete|","",""
"`LWG2767 <https://wg21.link/LWG2767>`__","not_fn call_wrapper can form invalid types","2016-11 (Issaquah)","|Complete|","",""
"`LWG2770 <https://wg21.link/LWG2770>`__","``tuple_size<const T>`` specialization is not SFINAE compatible and breaks decomposition declarations","2016-11 (Issaquah)","|Complete|","21",""
"`LWG2771 <https://wg21.link/LWG2771>`__","Broken Effects of some basic_string::compare functions in terms of basic_string_view","2016-11 (Issaquah)","|Complete|","",""
"`LWG2773 <https://wg21.link/LWG2773>`__","Making std::ignore constexpr","2016-11 (Issaquah)","|Complete|","",""
"`LWG2777 <https://wg21.link/LWG2777>`__","basic_string_view::copy should use char_traits::copy","2016-11 (Issaquah)","|Complete|","",""
13 changes: 5 additions & 8 deletions libcxx/include/__tuple/tuple_size.h
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Original file line number Diff line number Diff line change
@@ -32,20 +32,17 @@ template <class _Tp, class...>
using __enable_if_tuple_size_imp _LIBCPP_NODEBUG = _Tp;

template <class _Tp>
struct _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp< const _Tp,
__enable_if_t<!is_volatile<_Tp>::value>,
integral_constant<size_t, sizeof(tuple_size<_Tp>)>>>
struct _LIBCPP_TEMPLATE_VIS tuple_size<
__enable_if_tuple_size_imp<const _Tp, __enable_if_t<!is_volatile<_Tp>::value>, decltype(tuple_size<_Tp>::value)>>
: public integral_constant<size_t, tuple_size<_Tp>::value> {};

template <class _Tp>
struct _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp< volatile _Tp,
__enable_if_t<!is_const<_Tp>::value>,
integral_constant<size_t, sizeof(tuple_size<_Tp>)>>>
struct _LIBCPP_TEMPLATE_VIS tuple_size<
__enable_if_tuple_size_imp<volatile _Tp, __enable_if_t<!is_const<_Tp>::value>, decltype(tuple_size<_Tp>::value)>>
: public integral_constant<size_t, tuple_size<_Tp>::value> {};

template <class _Tp>
struct _LIBCPP_TEMPLATE_VIS
tuple_size<__enable_if_tuple_size_imp<const volatile _Tp, integral_constant<size_t, sizeof(tuple_size<_Tp>)>>>
struct _LIBCPP_TEMPLATE_VIS tuple_size<__enable_if_tuple_size_imp<const volatile _Tp, decltype(tuple_size<_Tp>::value)>>
: public integral_constant<size_t, tuple_size<_Tp>::value> {};

#else
8 changes: 5 additions & 3 deletions libcxx/test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_incomplete.pass.cpp
View file
Original file line number Diff line number Diff line change
@@ -45,9 +45,11 @@ void test_complete() {
template <class T>
void test_incomplete() {
static_assert(!is_complete<T>(), "");
static_assert(!is_complete<const T>(), "");
static_assert(!is_complete<volatile T>(), "");
static_assert(!is_complete<const volatile T>(), "");
// https://cplusplus.github.io/LWG/issue4040
// It is controversial whether these specializations are incomplete.
LIBCPP_STATIC_ASSERT(!is_complete<const T>(), "");
LIBCPP_STATIC_ASSERT(!is_complete<volatile T>(), "");
LIBCPP_STATIC_ASSERT(!is_complete<const volatile T>(), "");
}


30 changes: 27 additions & 3 deletions libcxx/test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_incomplete.verify.cpp
View file
Original file line number Diff line number Diff line change
@@ -23,6 +23,8 @@
struct Dummy1 {};
struct Dummy2 {};
struct Dummy3 {};
struct Dummy4 {};
struct Dummy5 {};

template <>
struct std::tuple_size<Dummy1> {
@@ -39,6 +41,16 @@ struct std::tuple_size<Dummy2> {
template <>
struct std::tuple_size<Dummy3> {};

template <>
struct std::tuple_size<Dummy4> {
void value();
};

template <>
struct std::tuple_size<Dummy5> {
size_t value;
};

void f() {
// Test that tuple_size<const T> is not incomplete when tuple_size<T>::value
// is well-formed but not a constant expression.
@@ -53,9 +65,21 @@ void f() {
(void)std::tuple_size<const Dummy2>::value; // expected-note {{here}}
}
// Test that tuple_size<const T> generates an error when tuple_size<T> is
// complete but ::value isn't a constant expression convertible to size_t.
// complete but has no ::value member.
{
// expected-error@*:* 1 {{implicit instantiation of undefined template}}
(void)std::tuple_size<const Dummy3>::value;
}
// Test that tuple_size<const T> generates an error when tuple_size<T> has
// the ::value member but tuple_size<T>::value is ill-formed.
{
// expected-error@*:* 1 {{implicit instantiation of undefined template}}
(void)std::tuple_size<const Dummy4>::value;
}
// Test that tuple_size<const T> generates an error when tuple_size<T> has
// the ::value member which is non-static.
{
// expected-error@*:* 1 {{no member named 'value'}}
(void)std::tuple_size<const Dummy3>::value; // expected-note {{here}}
// expected-error@*:* 1 {{invalid use of non-static data member 'value'}}
(void)std::tuple_size<const Dummy5>::value; // expected-note {{here}}
}
}
65 changes: 64 additions & 1 deletion ...test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_structured_bindings.pass.cpp
View file
Original file line number Diff line number Diff line change
@@ -109,6 +109,36 @@ void test_decomp_array() {
}
}

struct TestLWG2770 {
int n;
};

template <>
struct std::tuple_size<TestLWG2770> {};

void test_lwg_2770() {
{
auto [n] = TestLWG2770{42};
assert(n == 42);
}
{
const auto [n] = TestLWG2770{42};
assert(n == 42);
}
{
TestLWG2770 s{42};
auto& [n] = s;
assert(n == 42);
assert(&n == &s.n);
}
{
const TestLWG2770 s{42};
auto& [n] = s;
assert(n == 42);
assert(&n == &s.n);
}
}

struct Test {
int x;
};
@@ -136,16 +166,49 @@ struct std::tuple_size<Test> {
void test_after_tuple_size_specialization() {
Test const t{99};
auto& [p] = t;
assert(p == -1);
// https://cplusplus.github.io/LWG/issue4040
// It is controversial whether std::tuple_size<const Test> is instantiated here or before.
(void)p;
LIBCPP_ASSERT(p == -1);
}

#if TEST_STD_VER >= 26 && __cpp_structured_bindings >= 202411L
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Suggested change
#if TEST_STD_VER >= 26 && __cpp_structured_bindings >= 202411L
#if TEST_STD_VER >= 26

We normally just assume that TEST_STD_VER >= 26 implies that the language feature is available. Is there a reason to stray from that here?

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GCC hasn't implemented this feature yet, while I think it's worth testing empty structured bindings.

struct InvalidWhenNoCv1 {};

template <>
struct std::tuple_size<InvalidWhenNoCv1> {};

struct InvalidWhenNoCv2 {};

template <>
struct std::tuple_size<InvalidWhenNoCv2> {
void value();
};

template <class = void>
void test_decomp_as_empty_pack() {
{
const auto [... pack] = InvalidWhenNoCv1{};
static_assert(sizeof...(pack) == 0);
}
{
const auto [... pack] = InvalidWhenNoCv2{};
static_assert(sizeof...(pack) == 0);
}
}
#endif

int main(int, char**) {
test_decomp_user_type();
test_decomp_tuple();
test_decomp_pair();
test_decomp_array();
test_lwg_2770();
test_before_tuple_size_specialization();
test_after_tuple_size_specialization();
#if TEST_STD_VER >= 26 && __cpp_structured_bindings >= 202411L
test_decomp_as_empty_pack();
#endif

return 0;
}