[libc++][NFC] Fix include guard for decay_copy.h and remove underscores from the header

[ldionne]

Summary

We don't use double underscores for private header names when they are
in a subdirectory with double underscores already.

[• Quuxplusone]

I'm happy assuming buildkite is happy!
FWIW, I do see two adjacent open questions, neither of which needs to be addressed in this PR:

• Should <utility> include <__utility/decay_copy.h>? (I have no preference, except for consistency, and even gaining consistency seems like it'll be a bit of an uphill effort)
• Should decay_copy.h really have that // -*- C++ -*- on line 1? @Mordante has pointed out that the comment is useful (to certain syntax-highlighting editors) for files with no extension, such as <string>, but for .h and .cpp files I assume it has little benefit. Right now some __foo/*.h subdirectories are totally comment-less (__compare, __concepts, __random), some are totally comment-ful (__format, __iterator, __memory, __ranges), and some are mixed (__functional, __utility). Personally I prefer comment-less.

[ldionne]

In D108820#2969287, @Quuxplusone wrote:

I'm happy assuming buildkite is happy!
FWIW, I do see two adjacent open questions, neither of which needs to be addressed in this PR:

• Should <utility> include <__utility/decay_copy.h>? (I have no preference, except for consistency, and even gaining consistency seems like it'll be a bit of an uphill effort)

I guess it should, since we statued that <header> should include everything in <__header/*>. We can do that as a separate PR where we fix it for all subdirectories (you're welcome to do that!).

• Should decay_copy.h really have that // -*- C++ -*- on line 1? @Mordante has pointed out that the comment is useful (to certain syntax-highlighting editors) for files with no extension, such as <string>, but for .h and .cpp files I assume it has little benefit. Right now some __foo/*.h subdirectories are totally comment-less (__compare, __concepts, __random), some are totally comment-ful (__format, __iterator, __memory, __ranges), and some are mixed (__functional, __utility). Personally I prefer comment-less.

I dislike those comments too, however they have started proving useful for the extension-less since I'm using VS Code (for some reason, it doesn't guess the language otherwise). I think we should be consistent and use it everywhere.

libcxx/include/CMakeLists.txt

set(files
   __support/win32/locale_win32.h
   __support/xlocale/__nop_locale_mgmt.h
   __support/xlocale/__posix_l_fallback.h
   __support/xlocale/__strtonum_fallback.h
   __threading_support
   __tree
   __tuple
   __undef_macros
-  __utility/__decay_copy.h
   __utility/as_const.h
   __utility/cmp.h
+  __utility/decay_copy.h
   __utility/declval.h
   __utility/exchange.h
   __utility/forward.h
   __utility/in_place.h
   __utility/integer_sequence.h
   __utility/move.h
   __utility/pair.h
   __utility/piecewise_construct.h

libcxx/include/__ranges/access.h

 // -*- C++ -*-
 //===------------------------ __ranges/access.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___RANGES_ACCESS_H
 #define _LIBCPP___RANGES_ACCESS_H
 
 #include <__config>
 #include <__iterator/concepts.h>
 #include <__iterator/readable_traits.h>
 #include <__ranges/enable_borrowed_range.h>
-#include <__utility/__decay_copy.h>
 #include <__utility/as_const.h>
+#include <__utility/decay_copy.h>
 #include <__utility/forward.h>
 #include <concepts>
 #include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 

libcxx/include/__ranges/all.h

 #include <__config>
 #include <__iterator/concepts.h>
 #include <__iterator/iterator_traits.h>
 #include <__ranges/access.h>
 #include <__ranges/concepts.h>
 #include <__ranges/range_adaptor.h>
 #include <__ranges/ref_view.h>
 #include <__ranges/subrange.h>
-#include <__utility/__decay_copy.h>
+#include <__utility/decay_copy.h>
 #include <__utility/declval.h>
 #include <__utility/forward.h>
 #include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 

libcxx/include/__ranges/counted.h

 #include <__iterator/concepts.h>
 #include <__iterator/counted_iterator.h>
 #include <__iterator/default_sentinel.h>
 #include <__iterator/incrementable_traits.h>
 #include <__iterator/iterator_traits.h>
 #include <__memory/pointer_traits.h>
 #include <__ranges/concepts.h>
 #include <__ranges/subrange.h>
-#include <__utility/__decay_copy.h>
+#include <__utility/decay_copy.h>
 #include <__utility/declval.h>
 #include <__utility/forward.h>
 #include <__utility/move.h>
 #include <span>
 #include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header

libcxx/include/__ranges/size.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // 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___RANGES_SIZE_H
 #define _LIBCPP___RANGES_SIZE_H
 
 #include <__config>
 #include <__iterator/concepts.h>
 #include <__iterator/iterator_traits.h>
 #include <__ranges/access.h>
-#include <__utility/__decay_copy.h>
+#include <__utility/decay_copy.h>
 #include <__utility/forward.h>
 #include <concepts>
 #include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 

libcxx/include/__utility/decay_copy.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // 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_DECAY_COPY_H
-#define _LIBCPP___TYPE_TRAITS_DECAY_COPY_H
+#ifndef _LIBCPP___UTILITY_DECAY_COPY_H
+#define _LIBCPP___UTILITY_DECAY_COPY_H
 
 #include <__config>
 #include <__utility/forward.h>
 #include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif

 {
   return _VSTD::forward<_Tp>(__t);
 }
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
-#endif // _LIBCPP___TYPE_TRAITS_DECAY_COPY_H
+#endif // _LIBCPP___UTILITY_DECAY_COPY_H

libcxx/include/future

 
 */
 
 #include <__availability>
 #include <__config>
 #include <__debug>
 #include <__memory/allocator_arg_t.h>
 #include <__memory/uses_allocator.h>
-#include <__utility/__decay_copy.h>
+#include <__utility/decay_copy.h>
 #include <__utility/forward.h>
 #include <chrono>
 #include <exception>
 #include <memory>
 #include <mutex>
 #include <system_error>
 #include <thread>
 

libcxx/include/module.modulemap

module unordered_set {
     export *
   }
   module utility {
     header "utility"
     export initializer_list
     export *
 
     module __utility {
-      module __decay_copy        { private header "__utility/__decay_copy.h"        }
       module as_const            { private header "__utility/as_const.h"            }
       module cmp                 { private header "__utility/cmp.h"                 }
+      module decay_copy          { private header "__utility/decay_copy.h"          }
       module declval             { private header "__utility/declval.h"             }
       module exchange            { private header "__utility/exchange.h"            }
       module forward             { private header "__utility/forward.h"             }
       module in_place            { private header "__utility/in_place.h"            }
       module integer_sequence    { private header "__utility/integer_sequence.h"    }
       module move                { private header "__utility/move.h"                }
       module pair                { private header "__utility/pair.h"                }
       module piecewise_construct { private header "__utility/piecewise_construct.h" }

libcxx/include/thread

 
 */
 
 #include <__config>
 #include <__debug>
 #include <__functional_base>
 #include <__mutex_base>
 #include <__threading_support>
-#include <__utility/__decay_copy.h>
+#include <__utility/decay_copy.h>
 #include <__utility/forward.h>
 #include <chrono>
 #include <cstddef>
 #include <functional>
 #include <iosfwd>
 #include <memory>
 #include <system_error>
 #include <tuple>

libcxx/test/libcxx/diagnostics/detail.headers/utility/decay_copy.module.verify.cpp

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: modules-build
 
 // WARNING: This test was generated by 'generate_private_header_tests.py'
 // and should not be edited manually.
 
-// expected-error@*:* {{use of private header from outside its module: '__utility/__decay_copy.h'}}
-#include <__utility/__decay_copy.h>
+// expected-error@*:* {{use of private header from outside its module: '__utility/decay_copy.h'}}
+#include <__utility/decay_copy.h>