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[libc++] Properly decay functions in CTAD for pair #134544

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Apr 19, 2025
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[libc++] Properly decay functions in CTAD for pair #134544

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104 changes: 59 additions & 45 deletions libcxx/include/__utility/pair.h
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Original file line number Diff line number Diff line change
Expand Up @@ -49,6 +49,33 @@ _LIBCPP_PUSH_MACROS

_LIBCPP_BEGIN_NAMESPACE_STD

#ifndef _LIBCPP_CXX03_LANG
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question: isn't cxx03 using different header?

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Currently the combination of C++03 with non-frozen headers is still being tested in CI. I don't know when the support for this will be dropped.


template <class _T1, class _T2>
struct __check_pair_construction {
template <int&...>
static _LIBCPP_HIDE_FROM_ABI constexpr bool __enable_implicit_default() {
return __is_implicitly_default_constructible<_T1>::value && __is_implicitly_default_constructible<_T2>::value;
}

template <int&...>
static _LIBCPP_HIDE_FROM_ABI constexpr bool __enable_default() {
return is_default_constructible<_T1>::value && is_default_constructible<_T2>::value;
}

template <class _U1, class _U2>
static _LIBCPP_HIDE_FROM_ABI constexpr bool __is_pair_constructible() {
return is_constructible<_T1, _U1>::value && is_constructible<_T2, _U2>::value;
}

template <class _U1, class _U2>
static _LIBCPP_HIDE_FROM_ABI constexpr bool __is_implicit() {
return is_convertible<_U1, _T1>::value && is_convertible<_U2, _T2>::value;
}
};

#endif

template <class, class>
struct __non_trivially_copyable_base {
_LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI __non_trivially_copyable_base() _NOEXCEPT {}
Expand Down Expand Up @@ -104,40 +131,16 @@ struct _LIBCPP_TEMPLATE_VIS pair
return *this;
}
#else
struct _CheckArgs {
template <int&...>
static _LIBCPP_HIDE_FROM_ABI constexpr bool __enable_implicit_default() {
return __is_implicitly_default_constructible<_T1>::value && __is_implicitly_default_constructible<_T2>::value;
}

template <int&...>
static _LIBCPP_HIDE_FROM_ABI constexpr bool __enable_default() {
return is_default_constructible<_T1>::value && is_default_constructible<_T2>::value;
}

template <class _U1, class _U2>
static _LIBCPP_HIDE_FROM_ABI constexpr bool __is_pair_constructible() {
return is_constructible<first_type, _U1>::value && is_constructible<second_type, _U2>::value;
}

template <class _U1, class _U2>
static _LIBCPP_HIDE_FROM_ABI constexpr bool __is_implicit() {
return is_convertible<_U1, first_type>::value && is_convertible<_U2, second_type>::value;
}
};

template <bool _MaybeEnable>
using _CheckArgsDep _LIBCPP_NODEBUG = __conditional_t<_MaybeEnable, _CheckArgs, void>;

template <bool _Dummy = true, __enable_if_t<_CheckArgsDep<_Dummy>::__enable_default(), int> = 0>
explicit(!_CheckArgsDep<_Dummy>::__enable_implicit_default()) _LIBCPP_HIDE_FROM_ABI constexpr pair() noexcept(
template <class _CheckArgsDep = __check_pair_construction<_T1, _T2>,
__enable_if_t<_CheckArgsDep::__enable_default(), int> = 0>
explicit(!_CheckArgsDep::__enable_implicit_default()) _LIBCPP_HIDE_FROM_ABI constexpr pair() noexcept(
is_nothrow_default_constructible<first_type>::value && is_nothrow_default_constructible<second_type>::value)
: first(), second() {}

template <bool _Dummy = true,
__enable_if_t<_CheckArgsDep<_Dummy>::template __is_pair_constructible<_T1 const&, _T2 const&>(), int> = 0>
template <class _CheckArgsDep = __check_pair_construction<_T1, _T2>,
__enable_if_t<_CheckArgsDep::template __is_pair_constructible<_T1 const&, _T2 const&>(), int> = 0>
_LIBCPP_HIDE_FROM_ABI
_LIBCPP_CONSTEXPR_SINCE_CXX14 explicit(!_CheckArgsDep<_Dummy>::template __is_implicit<_T1 const&, _T2 const&>())
_LIBCPP_CONSTEXPR_SINCE_CXX14 explicit(!_CheckArgsDep::template __is_implicit<_T1 const&, _T2 const&>())
pair(_T1 const& __t1, _T2 const& __t2) noexcept(is_nothrow_copy_constructible<first_type>::value &&
is_nothrow_copy_constructible<second_type>::value)
: first(__t1), second(__t2) {}
Expand All @@ -150,41 +153,52 @@ struct _LIBCPP_TEMPLATE_VIS pair
class _U1,
class _U2,
# endif
__enable_if_t<_CheckArgs::template __is_pair_constructible<_U1, _U2>(), int> = 0 >
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I am trying to understand why the original code instantiate pair<T1, T2> at this point. is it because _CheckArgsDep was a member struct and it has to instantiate the enclosing class to be able to use it?

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I think that's the reason.

_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 explicit(!_CheckArgs::template __is_implicit<_U1, _U2>())
__enable_if_t<__check_pair_construction<_T1, _T2>::template __is_pair_constructible<_U1, _U2>(), int> = 0 >
_LIBCPP_HIDE_FROM_ABI
_LIBCPP_CONSTEXPR_SINCE_CXX14 explicit(!__check_pair_construction<_T1, _T2>::template __is_implicit<_U1, _U2>())
pair(_U1&& __u1, _U2&& __u2) noexcept(is_nothrow_constructible<first_type, _U1>::value &&
is_nothrow_constructible<second_type, _U2>::value)
: first(std::forward<_U1>(__u1)), second(std::forward<_U2>(__u2)) {
}

# if _LIBCPP_STD_VER >= 23
template <class _U1, class _U2, __enable_if_t<_CheckArgs::template __is_pair_constructible<_U1&, _U2&>(), int> = 0>
_LIBCPP_HIDE_FROM_ABI constexpr explicit(!_CheckArgs::template __is_implicit<_U1&, _U2&>())
template <class _U1,
class _U2,
__enable_if_t<__check_pair_construction<_T1, _T2>::template __is_pair_constructible<_U1&, _U2&>(), int> = 0>
_LIBCPP_HIDE_FROM_ABI constexpr explicit(!__check_pair_construction<_T1, _T2>::template __is_implicit<_U1&, _U2&>())
pair(pair<_U1, _U2>& __p) noexcept((is_nothrow_constructible<first_type, _U1&>::value &&
is_nothrow_constructible<second_type, _U2&>::value))
: first(__p.first), second(__p.second) {}
# endif

template <class _U1,
class _U2,
__enable_if_t<_CheckArgs::template __is_pair_constructible<_U1 const&, _U2 const&>(), int> = 0>
_LIBCPP_HIDE_FROM_ABI
_LIBCPP_CONSTEXPR_SINCE_CXX14 explicit(!_CheckArgs::template __is_implicit<_U1 const&, _U2 const&>())
template <
class _U1,
class _U2,
__enable_if_t<__check_pair_construction<_T1, _T2>::template __is_pair_constructible<_U1 const&, _U2 const&>(),
int> = 0>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 explicit(
!__check_pair_construction<_T1, _T2>::template __is_implicit<_U1 const&, _U2 const&>())
pair(pair<_U1, _U2> const& __p) noexcept(is_nothrow_constructible<first_type, _U1 const&>::value &&
is_nothrow_constructible<second_type, _U2 const&>::value)
: first(__p.first), second(__p.second) {}

template <class _U1, class _U2, __enable_if_t<_CheckArgs::template __is_pair_constructible<_U1, _U2>(), int> = 0>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 explicit(!_CheckArgs::template __is_implicit<_U1, _U2>())
template <class _U1,
class _U2,
__enable_if_t<__check_pair_construction<_T1, _T2>::template __is_pair_constructible<_U1, _U2>(), int> = 0>
_LIBCPP_HIDE_FROM_ABI
_LIBCPP_CONSTEXPR_SINCE_CXX14 explicit(!__check_pair_construction<_T1, _T2>::template __is_implicit<_U1, _U2>())
pair(pair<_U1, _U2>&& __p) noexcept(is_nothrow_constructible<first_type, _U1&&>::value &&
is_nothrow_constructible<second_type, _U2&&>::value)
: first(std::forward<_U1>(__p.first)), second(std::forward<_U2>(__p.second)) {}

# if _LIBCPP_STD_VER >= 23
template <class _U1,
class _U2,
__enable_if_t<_CheckArgs::template __is_pair_constructible<const _U1&&, const _U2&&>(), int> = 0>
_LIBCPP_HIDE_FROM_ABI constexpr explicit(!_CheckArgs::template __is_implicit<const _U1&&, const _U2&&>())
template <
class _U1,
class _U2,
__enable_if_t<__check_pair_construction<_T1, _T2>::template __is_pair_constructible<const _U1&&, const _U2&&>(),
int> = 0>
_LIBCPP_HIDE_FROM_ABI constexpr explicit(
!__check_pair_construction<_T1, _T2>::template __is_implicit<const _U1&&, const _U2&&>())
pair(const pair<_U1, _U2>&& __p) noexcept(is_nothrow_constructible<first_type, const _U1&&>::value &&
is_nothrow_constructible<second_type, const _U2&&>::value)
: first(std::move(__p.first)), second(std::move(__p.second)) {}
Expand Down
60 changes: 60 additions & 0 deletions libcxx/test/std/utilities/utility/pairs/pairs.pair/explicit_deduction_guides.pass.cpp
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Original file line number Diff line number Diff line change
@@ -0,0 +1,60 @@
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

// REQUIRES: std-at-least-c++17

// <utility>

// template<class T1, class T2>
// pair(T1, T2) -> pair<T1, T2>;

// Test that the explicit deduction guide for std::pair correctly decays function lvalues and
// behaves different from std::make_pair.

#include <cassert>
#include <functional>
#include <type_traits>
#include <utility>

#include "test_macros.h"

void dummy() {}

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could you please add a test for the test case in the github issue you've fixed?

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I think the first case in this test file is exactly for that GitHub issue. Did you mean adding a URL?

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NM. In the issue it was a lambda converted function pointer and here it is just regular functions.

constexpr void test_decay() {
char arr[1]{};
std::pair pr(arr, dummy);

ASSERT_SAME_TYPE(decltype(pr), std::pair<char*, void (*)()>);

assert(pr == std::make_pair(arr, dummy));
}

TEST_CONSTEXPR_CXX20 void test_unwrap() {
int n = 0;
std::pair pr(std::ref(n), dummy);

ASSERT_SAME_TYPE(decltype(pr), std::pair<std::reference_wrapper<int>, void (*)()>);
static_assert(!std::is_same_v<decltype(pr), decltype(std::make_pair(std::ref(n), dummy))>);

assert(&(pr.first.get()) == &n);
assert(pr.second == dummy);
}

constexpr bool test() {
test_decay();
if (TEST_STD_AT_LEAST_20_OR_RUNTIME_EVALUATED)
test_unwrap();
return true;
}

int main(int, char**) {
test();
static_assert(test());

return 0;
}