[libcxx-commits] [libcxx] Revert "[libc++][NFC] Simplify pair a bit" (PR #97003)

[James Y Knight via libcxx-commits]

https://github.com/jyknight created https://github.com/llvm/llvm-project/pull/97003

Reverts llvm/llvm-project#96165

The change broke code like
```
#include <utility>
#include <vector>

struct Test {
  std::vector<std::pair<int, Test>> v;
};

std::pair<int, Test> p;
```
under `-std=c++20`, apparently by triggering certain template evaluations too eagerly.

>From a2fb2a16f3d93364b8eaae82db443eb354299158 Mon Sep 17 00:00:00 2001
From: James Y Knight <foom at fuhm.net>
Date: Fri, 28 Jun 2024 01:02:40 -0400
Subject: [PATCH] Revert "[libc++][NFC] Simplify pair a bit (#96165)"

This reverts commit 54cb5ca9f48fc542b920662a0eee7c0e6f35bee0.
---
 libcxx/include/__utility/pair.h | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/libcxx/include/__utility/pair.h b/libcxx/include/__utility/pair.h
index 1b0d9241886f9..0afbebcdc9f2a 100644
--- a/libcxx/include/__utility/pair.h
+++ b/libcxx/include/__utility/pair.h
@@ -16,6 +16,8 @@
 #include <__fwd/array.h>
 #include <__fwd/pair.h>
 #include <__fwd/tuple.h>
+#include <__tuple/sfinae_helpers.h>
+#include <__tuple/tuple_element.h>
 #include <__tuple/tuple_indices.h>
 #include <__tuple/tuple_like_no_subrange.h>
 #include <__tuple/tuple_size.h>
@@ -128,15 +130,19 @@ struct _LIBCPP_TEMPLATE_VIS pair
     }
   };
 
-  template <bool _Dummy = true, __enable_if_t<_Dummy && _CheckArgs::__enable_default(), int> = 0>
-  explicit(!_CheckArgs::__enable_implicit_default()) _LIBCPP_HIDE_FROM_ABI constexpr pair() noexcept(
+  template <bool _MaybeEnable>
+  using _CheckArgsDep _LIBCPP_NODEBUG =
+      typename conditional< _MaybeEnable, _CheckArgs, __check_tuple_constructor_fail>::type;
+
+  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(
       is_nothrow_default_constructible<first_type>::value && is_nothrow_default_constructible<second_type>::value)
       : first(), second() {}
 
-  template <bool _Dummy                                                                                          = true,
-            __enable_if_t<_Dummy && _CheckArgs::template __is_pair_constructible<_T1 const&, _T2 const&>(), int> = 0>
+  template <bool _Dummy = true,
+            __enable_if_t<_CheckArgsDep<_Dummy>::template __is_pair_constructible<_T1 const&, _T2 const&>(), int> = 0>
   _LIBCPP_HIDE_FROM_ABI
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 explicit(!_CheckArgs::template __is_implicit<_T1 const&, _T2 const&>())
+  _LIBCPP_CONSTEXPR_SINCE_CXX14 explicit(!_CheckArgsDep<_Dummy>::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) {}