[libcxx-commits] [libcxx] [libc++] Simplify some meta programming in <variant> (PR #201538)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 created https://github.com/llvm/llvm-project/pull/201538

Specifically, this avoids `__dependent_type` and `__type_identity` instantiations, reducing compile times a bit.


>From 9c9114a6c764c7e1c6b463a54be5ddf8e3b4d228 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Thu, 4 Jun 2026 10:49:27 +0200
Subject: [PATCH] [libc++] Simplify some meta programming in <variant>

---
 libcxx/include/CMakeLists.txt                 |  1 -
 libcxx/include/__type_traits/dependent_type.h | 25 -------------------
 libcxx/include/module.modulemap.in            |  1 -
 libcxx/include/variant                        | 15 ++++++-----
 4 files changed, 7 insertions(+), 35 deletions(-)
 delete mode 100644 libcxx/include/__type_traits/dependent_type.h

diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index 10dfb4b4d58cb..e1da7aa279778 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -826,7 +826,6 @@ set(files
   __type_traits/copy_cvref.h
   __type_traits/datasizeof.h
   __type_traits/decay.h
-  __type_traits/dependent_type.h
   __type_traits/desugars_to.h
   __type_traits/detected_or.h
   __type_traits/disjunction.h
diff --git a/libcxx/include/__type_traits/dependent_type.h b/libcxx/include/__type_traits/dependent_type.h
deleted file mode 100644
index 354705c49c90e..0000000000000
--- a/libcxx/include/__type_traits/dependent_type.h
+++ /dev/null
@@ -1,25 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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_DEPENDENT_TYPE_H
-#define _LIBCPP___TYPE_TRAITS_DEPENDENT_TYPE_H
-
-#include <__config>
-
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
-
-_LIBCPP_BEGIN_NAMESPACE_STD
-
-template <class _Tp, bool>
-struct __dependent_type : public _Tp {};
-
-_LIBCPP_END_NAMESPACE_STD
-
-#endif // _LIBCPP___TYPE_TRAITS_DEPENDENT_TYPE_H
diff --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
index 2ccb455b7408d..0b141ecda1258 100644
--- a/libcxx/include/module.modulemap.in
+++ b/libcxx/include/module.modulemap.in
@@ -90,7 +90,6 @@ module std_core [system] {
     module copy_cvref                                 { header "__type_traits/copy_cvref.h" }
     module datasizeof                                 { header "__type_traits/datasizeof.h" }
     module decay                                      { header "__type_traits/decay.h" }
-    module dependent_type                             { header "__type_traits/dependent_type.h" }
     module desugars_to                                { header "__type_traits/desugars_to.h" }
     module detected_or                                { header "__type_traits/detected_or.h" }
     module disjunction                                { header "__type_traits/disjunction.h" }
diff --git a/libcxx/include/variant b/libcxx/include/variant
index 45462ee8a7713..95c93b814fbb8 100644
--- a/libcxx/include/variant
+++ b/libcxx/include/variant
@@ -235,7 +235,6 @@ namespace std {
 #  include <__type_traits/conditional.h>
 #  include <__type_traits/conjunction.h>
 #  include <__type_traits/decay.h>
-#  include <__type_traits/dependent_type.h>
 #  include <__type_traits/enable_if.h>
 #  include <__type_traits/invoke.h>
 #  include <__type_traits/is_array.h>
@@ -541,9 +540,12 @@ private:
     return __dispatcher<_Is...>::template __dispatch<_Fp, _Vs...>;
   }
 
+  template <class, class _Tp>
+  using __expander = _Tp;
+
   template <size_t _Ip, class _Fp, class... _Vs>
   _LIBCPP_HIDE_FROM_ABI static constexpr auto __make_fdiagonal_impl() {
-    return __make_dispatch<_Fp, _Vs...>(index_sequence<((void)__type_identity<_Vs>{}, _Ip)...>{});
+    return __make_dispatch<_Fp, _Vs...>(index_sequence<static_cast<__expander<_Vs, size_t>>(_Ip)...>{});
   }
 
   template <class _Fp, class... _Vs, size_t... _Is>
@@ -1167,8 +1169,7 @@ public:
   using __trivially_relocatable _LIBCPP_NODEBUG =
       conditional_t<_And<__libcpp_is_trivially_relocatable<_Types>...>::value, variant, void>;
 
-  template <bool _Dummy                                                                               = true,
-            enable_if_t<__dependent_type<is_default_constructible<__first_type>, _Dummy>::value, int> = 0>
+  template <class _FirstType = __first_type, enable_if_t<is_default_constructible_v<_FirstType>, int> = 0>
   _LIBCPP_HIDE_FROM_ABI constexpr variant() noexcept(is_nothrow_default_constructible_v<__first_type>)
       : __impl_(in_place_index<0>) {}
 
@@ -1283,10 +1284,8 @@ public:
 
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr size_t index() const noexcept { return __impl_.index(); }
 
-  template < bool _Dummy       = true,
-             enable_if_t< __all<(__dependent_type<is_move_constructible<_Types>, _Dummy>::value &&
-                                 __dependent_type<is_swappable<_Types>, _Dummy>::value)...>::value,
-                          int> = 0>
+  template <bool _Dummy = true,
+            enable_if_t<__all<_Dummy, (is_move_constructible_v<_Types> && is_swappable_v<_Types>)...>::value, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void swap(variant& __that) noexcept(
       __all<(is_nothrow_move_constructible_v<_Types> && is_nothrow_swappable_v<_Types>)...>::value) {
     __impl_.__swap(__that.__impl_);