1 files changed, 53 insertions, 0 deletions
diff --git a/libcxx/include/__type_traits/is_trivially_lexicographically_comparable.h b/libcxx/include/__type_traits/is_trivially_lexicographically_comparable.h
new file mode 100644
index 000000000000..a310ea1b87e3
--- /dev/null
+++ b/libcxx/include/__type_traits/is_trivially_lexicographically_comparable.h
@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_IS_TRIVIALLY_LEXICOGRAPHICALLY_COMPARABLE_H
+#define _LIBCPP___TYPE_TRAITS_IS_TRIVIALLY_LEXICOGRAPHICALLY_COMPARABLE_H
+
+#include <__config>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/is_unsigned.h>
+#include <__type_traits/remove_cv.h>
+#include <__type_traits/void_t.h>
+#include <__utility/declval.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// A type is_trivially_lexicographically_comparable if the expression `a <=> b` (or their pre-C++20 equivalents) is
+// equivalent to `std::memcmp(&a, &b, sizeof(T))` (with `a` and `b` being of type `T`). There is currently no builtin to
+// tell us whether that's the case for arbitrary types, so we can only do this for known types. Specifically, these are
+// currently unsigned integer types with a sizeof(T) == 1.
+//
+// bool is trivially lexicographically comparable, because e.g. false <=> true is valid code. Furthermore, the standard
+// says that [basic.fundamental] "Type bool is a distinct type that has the same object representation, value
+// representation, and alignment requirements as an implementation-defined unsigned integer type. The values of type
+// bool are true and false."
+// This means that bool has to be unsigned and has exactly two values. This means that having anything other than the
+// `true` or `false` value representations in a bool is UB.
+//
+// The following types are not trivially lexicographically comparable:
+// signed integer types: `char(-1) < char(1)`, but memcmp compares `unsigned char`s
+// unsigned integer types with sizeof(T) > 1: depending on the endianness, the LSB might be the first byte to be
+//                                            compared. This means that when comparing unsigned(129) and unsigned(2)
+//                                            using memcmp(), the result would be that 2 > 129.
+//                                            TODO: Do we want to enable this on big-endian systems?
+
+template <class _Tp, class _Up>
+struct __libcpp_is_trivially_lexicographically_comparable
+    : integral_constant<bool,
+                        is_same<__remove_cv_t<_Tp>, __remove_cv_t<_Up> >::value && sizeof(_Tp) == 1 &&
+                            is_unsigned<_Tp>::value> {};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___TYPE_TRAITS_IS_TRIVIALLY_LEXICOGRAPHICALLY_COMPARABLE_H