[libc++] Improve the implementation of std::unreachable

[ldionne]

Summary

First, use __builtin_unreachable unconditionally. It is implemented by
all the compilers that we support. Clang started supporting it around
Clang 4, and GCC around GCC 4.10.

Also add _LIBCPP_ASSERT so that we will actually get a guaranteed crash
if we reached std::unreachable() and assertions have been enabled,
since that's UB that's extremely easy to catch.

[philnik]

Do we want to abort when in debug mode? While it doesn't give the speed improvements in debug mode it would be really nice to get informed when std::unreachable is reached.

[huixie90]

LGTM. I would leave approvals to others

[ldionne]

Rebase onto main and address comments

libcxx/include/__utility/unreachable.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___UTILITY_UNREACHABLE_H
 #define _LIBCPP___UTILITY_UNREACHABLE_H
 
+#include <__assert>
 #include <__config>
-#include <cstdlib>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-_LIBCPP_NORETURN _LIBCPP_HIDE_FROM_ABI inline void __libcpp_unreachable()
-{
-#if __has_builtin(__builtin_unreachable)
+_LIBCPP_NORETURN _LIBCPP_HIDE_FROM_ABI inline void __libcpp_unreachable() {
+    _LIBCPP_ASSERT(false, "std::unreachable() was reached");
     __builtin_unreachable();
-#else
-  std::abort();
-#endif
 }
 
 #if _LIBCPP_STD_VER > 20
 
 [[noreturn]] _LIBCPP_HIDE_FROM_ABI inline void unreachable() { __libcpp_unreachable(); }
 
-#endif // _LIBCPP_STD_VER > 20
+#endif
 
 _LIBCPP_END_NAMESPACE_STD
 
-#endif
+#endif // _LIBCPP___UTILITY_UNREACHABLE_H

libcxx/include/utility

 #include <__tuple_dir/tuple_element.h>
 #include <__tuple_dir/tuple_size.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
+#  include <cstdlib>
 #  include <iosfwd>
 #  include <type_traits>
 #endif
 
 #endif // _LIBCPP_UTILITY

libcxx/test/libcxx/transitive_includes/cxx2b.csv

 unordered_set new
 unordered_set optional
 unordered_set stdexcept
 unordered_set tuple
 unordered_set type_traits
 unordered_set version
 utility compare
 utility cstddef
-utility cstdlib
 utility initializer_list
 utility limits
 utility version
 valarray cmath
 valarray cstddef
 valarray cstdlib
 valarray cstring
 valarray initializer_list

libcxx/test/std/utilities/utility/utility.unreachable/assert.unreachable.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// REQUIRES: has-unix-headers
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11.0|12.0}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+// Make sure that reaching std::unreachable() with assertions enabled triggers an assertion.
+
+#include <utility>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+    TEST_LIBCPP_ASSERT_FAILURE(std::unreachable(), "std::unreachable() was reached");
+
+    return 0;
+}

libcxx/test/std/utilities/utility/utility.unreachable/unreachable.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
 
-#include <utility>
+// Make sure we can use `std::unreachable()`. We can't actually execute it cause that would be
+// UB, but we can make sure that it doesn't cause a linker error or something like that.
+
+#include <cassert>
 #include <type_traits>
+#include <utility>
+
+int main(int argc, char**) {
+    assert(argc == 1);
+    if (argc != 1) {
+        std::unreachable();
+    }
 
     static_assert(std::is_same_v<decltype(std::unreachable()), void>);
+
+    return 0;
+}