[libc++] Revert a use of `static_cast` for `_VSTD::forward`. NFCI

[• Quuxplusone]

Summary

As requested in D107584.

[ldionne]

Can we revert all the functional changes that were made in D107584? It's really a matter of principle - if we start making changes in modularization patches (or any NFCI patch that is large enough to be difficult to audit), we lose an important contract with the reviewer that makes reviewing these patches palatable.

I'm 100% open to reviewing these "functional" changes on their own, but I just can't reasonably review them alongside 1000+ lines of NFC changes.

[• Quuxplusone]

Can we revert all the functional changes that were made in D107584?

Unfortunately, I'm not aware of any functional changes that were made in D107584. I see a bunch of repetitive comments from Chris on that review, but I don't see what change he's referring to; the left-hand and right-hand sides look the same to me. So I'm not going to pursue this any further. If people want specific changes made, relative to the status quo, I think they should be specific about them.

[ldionne]

In D108743#2967260, @Quuxplusone wrote:

Can we revert all the functional changes that were made in D107584?

Unfortunately, I'm not aware of any functional changes that were made in D107584. I see a bunch of repetitive comments from Chris on that review, but I don't see what change he's referring to; the left-hand and right-hand sides look the same to me. So I'm not going to pursue this any further. If people want specific changes made, relative to the status quo, I think they should be specific about them.

Oh crap, you're right, the other flagged changes appear to be whitespace changes only. Sorry about that. Please ship this!

libcxx/include/__concepts/boolean_testable.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___CONCEPTS_BOOLEAN_TESTABLE_H
 #define _LIBCPP___CONCEPTS_BOOLEAN_TESTABLE_H
 
 #include <__concepts/convertible_to.h>
 #include <__config>
+#include <__utility/forward.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)
 
 // [concepts.booleantestable]
 
 template<class _Tp>
 concept __boolean_testable_impl = convertible_to<_Tp, bool>;
 
 template<class _Tp>
 concept __boolean_testable = __boolean_testable_impl<_Tp> && requires(_Tp&& __t) {
-  { !static_cast<_Tp&&>(__t) } -> __boolean_testable_impl;
+  { !_VSTD::forward<_Tp>(__t) } -> __boolean_testable_impl;
 };
 
 #endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___CONCEPTS_BOOLEAN_TESTABLE_H