From 3460b30fd5c13eaccb7bcb1d81195654ea17181f Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@meta.com>
Date: Sat, 11 May 2024 18:58:40 -0700
Subject: [PATCH] Improve utf-8 detection

---
 include/fmt/base.h  |  8 ++++++--
 include/fmt/xchar.h | 12 +++++-------
 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/include/fmt/base.h b/include/fmt/base.h
index 2de369f5..aef46612 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -420,7 +420,7 @@ struct is_std_string_like<T, void_t<decltype(std::declval<T>().find_first_of(
                                  typename T::value_type(), 0))>>
     : std::true_type {};
 
-FMT_CONSTEXPR inline auto is_utf8() -> bool {
+FMT_CONSTEXPR inline auto is_utf8_enabled() -> bool {
   FMT_MSC_WARNING(suppress : 4566) constexpr unsigned char section[] = "\u00A7";
   // Avoid an MSVC sign extension bug: https://github.com/fmtlib/fmt/pull/2297.
   using uchar = unsigned char;
@@ -428,7 +428,11 @@ FMT_CONSTEXPR inline auto is_utf8() -> bool {
                         uchar(section[1]) == 0xA7;
   static_assert(utf8 || !FMT_MSC_VERSION,
                 "Unicode support requires compiling with /utf-8");
-  return FMT_UNICODE || utf8;
+  return utf8;
+}
+
+FMT_CONSTEXPR inline auto is_utf8() -> bool {
+  return FMT_UNICODE || is_utf8_enabled();
 }
 
 template <typename Char> FMT_CONSTEXPR auto length(const Char* s) -> size_t {
diff --git a/include/fmt/xchar.h b/include/fmt/xchar.h
index a6bf47fe..aa695582 100644
--- a/include/fmt/xchar.h
+++ b/include/fmt/xchar.h
@@ -23,12 +23,6 @@
 FMT_BEGIN_NAMESPACE
 namespace detail {
 
-#ifdef __cpp_char8_t
-using char8_type = char8_t;
-#else
-enum char8_type : unsigned char {};
-#endif
-
 template <typename T>
 using is_exotic_char = bool_constant<!std::is_same<T, char>::value>;
 
@@ -83,10 +77,14 @@ inline auto runtime(wstring_view s) -> runtime_format_string<wchar_t> {
 #endif
 
 template <> struct is_char<wchar_t> : std::true_type {};
-template <> struct is_char<detail::char8_type> : std::true_type {};
 template <> struct is_char<char16_t> : std::true_type {};
 template <> struct is_char<char32_t> : std::true_type {};
 
+#ifdef __cpp_char8_t
+template <>
+struct is_char<char8_t> : bool_constant<detail::is_utf8_enabled()> {};
+#endif
+
 template <typename... T>
 constexpr auto make_wformat_args(T&... args)
     -> decltype(fmt::make_format_args<wformat_context>(args...)) {