Specialize is_char for character types

include/fmt/core.h

@@ -443,7 +443,11 @@ using string_view = basic_string_view<char>;
 using wstring_view = basic_string_view<wchar_t>;
 
 /** Specifies if ``T`` is a character type. Can be specialized by users. */
-template <typename T> struct is_char : std::is_integral<T> {};
+template <typename T> struct is_char : std::false_type {};
+template <> struct is_char<char> : std::true_type {};
+template <> struct is_char<wchar_t> : std::true_type {};
+template <> struct is_char<char16_t> : std::true_type {};
+template <> struct is_char<char32_t> : std::true_type {};
 
 /**
   \rst

include/fmt/format.h

@@ -400,6 +400,7 @@ void buffer<T>::append(const U* begin, const U* end) {
 // A UTF-8 code unit type.
 enum char8_t : unsigned char {};
 #endif
+template <> struct is_char<char8_t> : std::true_type {};
 
 // A UTF-8 string view.
 class u8string_view : public basic_string_view<char8_t> {