[libc++] Make chars_format a bitmask type.

Some of Microsoft's unit tests in D70631 fail because libc++'s
implementation of std::chars_format isn't a proper bitmask type. Adding
the required functions to make std::chars_format a proper bitmask type.

Implements parts of P0067: Elementary string conversions

Differential Revision: https://reviews.llvm.org/D97115

libcxx/include/charconv

@@ -81,6 +81,7 @@ namespace std {
 #include <cstring>
 #include <limits>
 #include <type_traits>
+#include <utility>
 
 #include <__debug>
 
@@ -108,6 +109,47 @@ enum class _LIBCPP_ENUM_VIS chars_format
     general = fixed | scientific
 };
 
+inline _LIBCPP_INLINE_VISIBILITY constexpr chars_format
+operator~(chars_format __x) {
+  return chars_format(~_VSTD::__to_underlying(__x));
+}
+
+inline _LIBCPP_INLINE_VISIBILITY constexpr chars_format
+operator&(chars_format __x, chars_format __y) {
+  return chars_format(_VSTD::__to_underlying(__x) &
+                      _VSTD::__to_underlying(__y));
+}
+
+inline _LIBCPP_INLINE_VISIBILITY constexpr chars_format
+operator|(chars_format __x, chars_format __y) {
+  return chars_format(_VSTD::__to_underlying(__x) |
+                      _VSTD::__to_underlying(__y));
+}
+
+inline _LIBCPP_INLINE_VISIBILITY constexpr chars_format
+operator^(chars_format __x, chars_format __y) {
+  return chars_format(_VSTD::__to_underlying(__x) ^
+                      _VSTD::__to_underlying(__y));
+}
+
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11 chars_format&
+operator&=(chars_format& __x, chars_format __y) {
+  __x = __x & __y;
+  return __x;
+}
+
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11 chars_format&
+operator|=(chars_format& __x, chars_format __y) {
+  __x = __x | __y;
+  return __x;
+}
+
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11 chars_format&
+operator^=(chars_format& __x, chars_format __y) {
+  __x = __x ^ __y;
+  return __x;
+}
+
 struct _LIBCPP_TYPE_VIS to_chars_result
 {
     char* ptr;

libcxx/test/std/utilities/charconv/charconv.syn/chars_format.pass.cpp

@@ -0,0 +1,78 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// Note: chars_format is a C++17 feature backported to C++11. Assert isn't
+// allowed in a constexpr function in C++11. To keep the code readable, C++11
+// support is untested.
+// UNSUPPORTED: c++03, c++11
+
+// <charconv>
+
+// Bitmask type
+// enum class chars_format {
+//   scientific = unspecified,
+//   fixed = unspecified,
+//   hex = unspecified,
+//   general = fixed | scientific
+// };
+
+#include <charconv>
+#include <cassert>
+
+#include "test_macros.h"
+
+constexpr bool test() {
+  using cf = std::chars_format;
+  using ut = std::underlying_type<cf>::type;
+
+  {
+    cf x = cf::scientific;
+    x |= cf::fixed;
+    assert(x == cf::general);
+  }
+  {
+    cf x = cf::general;
+    x &= cf::fixed;
+    assert(x == cf::fixed);
+  }
+  {
+    cf x = cf::general;
+    x ^= cf::fixed;
+    assert(x == cf::scientific);
+  }
+
+  assert(static_cast<ut>(cf::scientific & (cf::fixed | cf::hex)) == 0);
+  assert(static_cast<ut>(cf::fixed & (cf::scientific | cf::hex)) == 0);
+  assert(static_cast<ut>(cf::hex & (cf::scientific | cf::fixed)) == 0);
+
+  assert((cf::scientific | cf::fixed) == cf::general);
+
+  assert(static_cast<ut>(cf::scientific & cf::fixed) == 0);
+
+  assert((cf::general ^ cf::fixed) == cf::scientific);
+
+  assert((~cf::hex & cf::general) == cf::general);
+
+  return true;
+}
+
+std::chars_format x;
+static_assert(std::is_same<std::chars_format, decltype(~x)>::value, "");
+static_assert(std::is_same<std::chars_format, decltype(x & x)>::value, "");
+static_assert(std::is_same<std::chars_format, decltype(x | x)>::value, "");
+static_assert(std::is_same<std::chars_format, decltype(x ^ x)>::value, "");
+static_assert(std::is_same<std::chars_format&, decltype(x &= x)>::value, "");
+static_assert(std::is_same<std::chars_format&, decltype(x |= x)>::value, "");
+static_assert(std::is_same<std::chars_format&, decltype(x ^= x)>::value, "");
+
+int main(int, char**) {
+  assert(test());
+  static_assert(test(), "");
+
+  return 0;
+}