Revert "[libc++][NFC] Simplify pair a bit"

[jyknight]

Reverts #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.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: James Y Knight (jyknight)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/97003.diff

1 Files Affected:

• (modified) libcxx/include/__utility/pair.h (+11-5)

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) {}

[philnik777]

Could you add the test case to avoid regressing on this again?

[alexfh]

LG. I think, it's better to add a test separately from the revert.

[ldionne]

LGTM. I'll merge this to get back to a clean state ASAP, but @jyknight can you please submit a PR to add this regression test?

[jyknight]

Created PR #97355 with a test.