[llvm-branch-commits] [libcxx] release/20.x: Revert "[libc++] Reduce std::conjunction overhead (#124259)" (PR #127677)

[via llvm-branch-commits]

https://github.com/llvmbot updated https://github.com/llvm/llvm-project/pull/127677

>From 04295354b0f0f101773c4f9437680d45a70bab24 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Fri, 7 Feb 2025 15:40:16 +0100
Subject: [PATCH] Revert "[libc++] Reduce std::conjunction overhead (#124259)"

It turns out that the new implementation takes significantly more stack
memory for some reason.

This reverts commit 2696e4fb9567d23ce065a067e7f4909b310daf50.

(cherry picked from commit 0227396417d4625bc93affdd8957ff8d90c76299)
---
 libcxx/include/__type_traits/conjunction.h | 42 ++++++++++++----------
 1 file changed, 24 insertions(+), 18 deletions(-)

diff --git a/libcxx/include/__type_traits/conjunction.h b/libcxx/include/__type_traits/conjunction.h
index 6b6717a50a468..ad9656acd47ec 100644
--- a/libcxx/include/__type_traits/conjunction.h
+++ b/libcxx/include/__type_traits/conjunction.h
@@ -10,6 +10,8 @@
 #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>
 #include <__type_traits/is_same.h>
 
@@ -19,29 +21,22 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <bool>
-struct _AndImpl;
+template <class...>
+using __expand_to_true _LIBCPP_NODEBUG = true_type;
 
-template <>
-struct _AndImpl<true> {
-  template <class _Res, class _First, class... _Rest>
-  using _Result _LIBCPP_NODEBUG =
-      typename _AndImpl<bool(_First::value) && sizeof...(_Rest) != 0>::template _Result<_First, _Rest...>;
-};
+template <class... _Pred>
+__expand_to_true<__enable_if_t<_Pred::value>...> __and_helper(int);
 
-template <>
-struct _AndImpl<false> {
-  template <class _Res, class...>
-  using _Result _LIBCPP_NODEBUG = _Res;
-};
+template <class...>
+false_type __and_helper(...);
 
 // _And always performs lazy evaluation of its arguments.
 //
 // However, `_And<_Pred...>` itself will evaluate its result immediately (without having to
 // be instantiated) since it is an alias, unlike `conjunction<_Pred...>`, which is a struct.
 // If you want to defer the evaluation of `_And<_Pred...>` itself, use `_Lazy<_And, _Pred...>`.
-template <class... _Args>
-using _And _LIBCPP_NODEBUG = typename _AndImpl<sizeof...(_Args) != 0>::template _Result<true_type, _Args...>;
+template <class... _Pred>
+using _And _LIBCPP_NODEBUG = decltype(std::__and_helper<_Pred...>(0));
 
 template <bool... _Preds>
 struct __all_dummy;
@@ -51,11 +46,22 @@ struct __all : _IsSame<__all_dummy<_Pred...>, __all_dummy<((void)_Pred, true)...
 
 #if _LIBCPP_STD_VER >= 17
 
-template <class... _Args>
-struct _LIBCPP_NO_SPECIALIZATIONS conjunction : _And<_Args...> {};
+template <class...>
+struct _LIBCPP_NO_SPECIALIZATIONS conjunction : true_type {};
+
+_LIBCPP_DIAGNOSTIC_PUSH
+#  if __has_warning("-Winvalid-specialization")
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Winvalid-specialization")
+#  endif
+template <class _Arg>
+struct conjunction<_Arg> : _Arg {};
+
+template <class _Arg, class... _Args>
+struct conjunction<_Arg, _Args...> : conditional_t<!bool(_Arg::value), _Arg, conjunction<_Args...>> {};
+_LIBCPP_DIAGNOSTIC_POP
 
 template <class... _Args>
-_LIBCPP_NO_SPECIALIZATIONS inline constexpr bool conjunction_v = _And<_Args...>::value;
+_LIBCPP_NO_SPECIALIZATIONS inline constexpr bool conjunction_v = conjunction<_Args...>::value;
 
 #endif // _LIBCPP_STD_VER >= 17