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

[philnik777]

No description provided.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/96165.diff

1 Files Affected:

• (modified) libcxx/include/__utility/pair.h (+5-12)

diff --git a/libcxx/include/__utility/pair.h b/libcxx/include/__utility/pair.h
index 1f5d19dd4c2b1..8872e5d9bef58 100644
--- a/libcxx/include/__utility/pair.h
+++ b/libcxx/include/__utility/pair.h
@@ -16,8 +16,6 @@
 #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>
@@ -130,20 +128,15 @@ struct _LIBCPP_TEMPLATE_VIS pair
     }
   };
 
-  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
-  _LIBCPP_CONSTEXPR pair() _NOEXCEPT_(
+  template <bool _Dummy = true, __enable_if_t<_Dummy && _CheckArgs::__enable_default(), int> = 0>
+  explicit(!_CheckArgs::__enable_implicit_default()) _LIBCPP_HIDE_FROM_ABI _LIBCPP_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 <bool _Dummy                                                                                          = true,
+            __enable_if_t<_Dummy && _CheckArgs::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&>()) pair(_T1 const& __t1, _T2 const& __t2)
+      !_CheckArgs::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) {}
 

[ldionne]

LGTM if CI passes, but I am pretty sure the code was written that way because this was upsetting compilers at some point. The condition inside explicit is technically not dependent as written, and I think this was causing problems.

Again, if CI happy, I am happy.

[jyknight]

I believe this change is broken. This following code now fails to compile, under clang -std=c++20 -stdlib=libc++, and I think it ought not to.

#include <utility>
#include <vector>

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

std::pair<int, Test> p;