[libc++] Disentangle _If, _Or and _And

[ldionne]

This LGTM once CI is passing (unless fixing the CI requires changing the approach taken by this patch, in which case please ping me for a second look).

[ldionne]

libcxx/include/__type_traits/conjunction.h

27

I imagine it should be possible to optimize this similarly to what we're doing with disjunction below?

[EricWF]

Frankly, I'm not sure that keeping logical operators together is "disentangling" them. They're pretty tightly grouped.
And I can't say understand the point or value of all this churn either.

But on a purely technical level, this change LGTM.

libcxx/include/__type_traits/conjunction.h

53

ADL.

[philnik]

Rebased

[philnik]

• Try to fix CI

[philnik]

• Change the name

[philnik]

• Use conditional for conditional_t

libcxx/include/__type_traits/conditional.h

 #include <__config>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+template <bool>
+struct _IfImpl;
+
+template <>
+struct _IfImpl<true> {
+  template <class _If, class _Else>
+  using _Select _LIBCPP_NODEBUG = _If;
+};
+
+template <>
+struct _IfImpl<false> {
+  template <class _If, class _Else>
+  using _Select _LIBCPP_NODEBUG = _Else;
+};
+
+template <bool _Cond, class _IfRes, class _ElseRes>
+using _If _LIBCPP_NODEBUG = typename _IfImpl<_Cond>::template _Select<_IfRes, _ElseRes>;
+
 template <bool _Bp, class _If, class _Then>
     struct _LIBCPP_TEMPLATE_VIS conditional {typedef _If type;};
 template <class _If, class _Then>
     struct _LIBCPP_TEMPLATE_VIS conditional<false, _If, _Then> {typedef _Then type;};
 
 #if _LIBCPP_STD_VER > 11
-template <bool _Bp, class _If, class _Then> using conditional_t = typename conditional<_Bp, _If, _Then>::type;
+template <bool _Bp, class _IfRes, class _ElseRes>
+using conditional_t = _If<_Bp, _IfRes, _ElseRes>;
 #endif
 
 // Helper so we can use "conditional_t" in all language versions.
 template <bool _Bp, class _If, class _Then> using __conditional_t = typename conditional<_Bp, _If, _Then>::type;
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___TYPE_TRAITS_CONDITIONAL_H

libcxx/include/__type_traits/conjunction.h

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___TYPE_TRAITS_CONJUNCTION_H
 #define _LIBCPP___TYPE_TRAITS_CONJUNCTION_H
 
 #include <__config>
 #include <__type_traits/conditional.h>
+#include <__type_traits/enable_if.h>
 #include <__type_traits/integral_constant.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
-#if _LIBCPP_STD_VER > 14
-
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#if _LIBCPP_STD_VER > 14
+
 template <class _Arg, class... _Args>
 struct __conjunction_impl {
   using type = conditional_t<!bool(_Arg::value), _Arg, typename __conjunction_impl<_Args...>::type>;
 };
 
 template <class _Arg>
 struct __conjunction_impl<_Arg> {
   using type = _Arg;
 };
 
 template <class... _Args>
 struct conjunction : __conjunction_impl<true_type, _Args...>::type {};
 
 template<class... _Args>
 inline constexpr bool conjunction_v = conjunction<_Args...>::value;
 
-_LIBCPP_END_NAMESPACE_STD
-
 #endif // _LIBCPP_STD_VER > 14
 
+template <class...>
+using __expand_to_true = true_type;
+
+template <class... _Pred>
+__expand_to_true<__enable_if_t<_Pred::value>...> __and_helper(int);
+
+template <class...>
+false_type __and_helper(...);
+
+template <class... _Pred>
+using _And _LIBCPP_NODEBUG = decltype(__and_helper<_Pred...>(0));
+
+_LIBCPP_END_NAMESPACE_STD
+
 #endif // _LIBCPP___TYPE_TRAITS_CONJUNCTION_H

libcxx/include/__type_traits/disjunction.h

 #include <__config>
 #include <__type_traits/conditional.h>
 #include <__type_traits/integral_constant.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
-#if _LIBCPP_STD_VER > 14
-
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _Arg, class... _Args>
-struct __disjunction_impl {
-  using type = conditional_t<bool(_Arg::value), _Arg, typename __disjunction_impl<_Args...>::type>;
+template <bool>
+struct _OrImpl;
+
+template <>
+struct _OrImpl<true> {
+  template <class _Res, class _First, class... _Rest>
+  using _Result _LIBCPP_NODEBUG =
+      typename _OrImpl<!bool(_First::value) && sizeof...(_Rest) != 0>::template _Result<_First, _Rest...>;
 };
 
-template <class _Arg>
-struct __disjunction_impl<_Arg> {
-  using type = _Arg;
+template <>
+struct _OrImpl<false> {
+  template <class _Res, class...>
+  using _Result = _Res;
 };
 
 template <class... _Args>
-struct disjunction : __disjunction_impl<false_type, _Args...>::type {};
-template<class... _Args>
-inline constexpr bool disjunction_v = disjunction<_Args...>::value;
+using _Or _LIBCPP_NODEBUG = typename _OrImpl<sizeof...(_Args) != 0>::template _Result<false_type, _Args...>;
+
+#if _LIBCPP_STD_VER > 14
+
+template <class... _Args>
+struct disjunction : _Or<_Args...> {};
 
-_LIBCPP_END_NAMESPACE_STD
+template <class... _Args>
+inline constexpr bool disjunction_v = _Or<_Args...>::value;
 
 #endif // _LIBCPP_STD_VER > 14
 
+_LIBCPP_END_NAMESPACE_STD
+
 #endif // _LIBCPP___TYPE_TRAITS_DISJUNCTION_H

libcxx/include/type_traits

 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _T1, class _T2> struct _LIBCPP_TEMPLATE_VIS pair;
 template <class _Tp> struct _LIBCPP_TEMPLATE_VIS hash;
 
 template <bool> struct _MetaBase;
 template <>
 struct _MetaBase<true> {
-  template <class _Tp, class _Up>
-  using _SelectImpl _LIBCPP_NODEBUG = _Tp;
   template <template <class...> class _FirstFn, template <class...> class, class ..._Args>
   using _SelectApplyImpl _LIBCPP_NODEBUG = _FirstFn<_Args...>;
-  template <class _Result, class _First, class ..._Rest>
-  using _OrImpl _LIBCPP_NODEBUG = typename _MetaBase<_First::value != true && sizeof...(_Rest) != 0>::template _OrImpl<_First, _Rest...>;
+  template <class _First, class...>
+  using _FirstImpl _LIBCPP_NODEBUG = _First;
+  template <class, class _Second, class...>
+  using _SecondImpl _LIBCPP_NODEBUG = _Second;
 };
 
 template <>
 struct _MetaBase<false> {
-  template <class _Tp, class _Up>
-  using _SelectImpl _LIBCPP_NODEBUG = _Up;
   template <template <class...> class, template <class...> class _SecondFn, class ..._Args>
   using _SelectApplyImpl _LIBCPP_NODEBUG = _SecondFn<_Args...>;
-  template <class _Result, class ...>
-  using _OrImpl _LIBCPP_NODEBUG = _Result;
 };
-template <bool _Cond, class _IfRes, class _ElseRes>
-using _If _LIBCPP_NODEBUG = typename _MetaBase<_Cond>::template _SelectImpl<_IfRes, _ElseRes>;
-template <class ..._Rest>
-using _Or _LIBCPP_NODEBUG = typename _MetaBase< sizeof...(_Rest) != 0 >::template _OrImpl<false_type, _Rest...>;
-
-template <class ...> using __expand_to_true = true_type;
-template <class ..._Pred>
-__expand_to_true<__enable_if_t<_Pred::value>...> __and_helper(int);
-template <class ...>
-false_type __and_helper(...);
-template <class ..._Pred>
-using _And _LIBCPP_NODEBUG = decltype(__and_helper<_Pred...>(0));
+
+template <class ..._Args>
+using _FirstType _LIBCPP_NODEBUG = typename _MetaBase<(sizeof...(_Args) >= 1)>::template _FirstImpl<_Args...>;
+template <class ..._Args>
+using _SecondType _LIBCPP_NODEBUG = typename _MetaBase<(sizeof...(_Args) >= 2)>::template _SecondImpl<_Args...>;
 
 template <template <class...> class _Func, class ..._Args>
 struct _Lazy : _Func<_Args...> {};
 
 // Member detector base
 
 template <template <class...> class _Templ, class ..._Args, class = _Templ<_Args...> >
 true_type __sfinae_test_impl(int);