Refactor problematic trailing returns in arg_mapper

include/fmt/core.h

@@ -8,8 +8,8 @@
 #ifndef FMT_CORE_H_
 #define FMT_CORE_H_
 
-#include <cstddef> // std::byte
-#include <cstdio>  // std::FILE
+#include <cstddef>  // std::byte
+#include <cstdio>   // std::FILE
 #include <cstring>
 #include <iterator>
 #include <limits>
@@ -1364,20 +1364,21 @@ template <typename Context> struct arg_mapper {
       -> basic_string_view<char_type> {
     return std_string_view<char_type>(val);
   }
-  FMT_CONSTEXPR FMT_INLINE auto map(const signed char* val)
-      -> decltype(this->map("")) {
+
+  using cstring_result = conditional_t<std::is_same<char_type, char>::value,
+                                       const char*, unformattable_pointer>;
+
+  FMT_CONSTEXPR FMT_INLINE auto map(const signed char* val) -> cstring_result {
     return map(reinterpret_cast<const char*>(val));
   }
   FMT_CONSTEXPR FMT_INLINE auto map(const unsigned char* val)
-      -> decltype(this->map("")) {
+      -> cstring_result {
     return map(reinterpret_cast<const char*>(val));
   }
-  FMT_CONSTEXPR FMT_INLINE auto map(signed char* val)
-      -> decltype(this->map("")) {
+  FMT_CONSTEXPR FMT_INLINE auto map(signed char* val) -> cstring_result {
     return map(reinterpret_cast<const char*>(val));
   }
-  FMT_CONSTEXPR FMT_INLINE auto map(unsigned char* val)
-      -> decltype(this->map("")) {
+  FMT_CONSTEXPR FMT_INLINE auto map(unsigned char* val) -> cstring_result {
     return map(reinterpret_cast<const char*>(val));
   }
 
@@ -1410,9 +1411,8 @@ template <typename Context> struct arg_mapper {
 
   template <typename T,
             FMT_ENABLE_IF(std::is_enum<T>::value &&
-                          (std::is_convertible<T, int>::value
-                           || std::is_same<T, detail::byte>::value
-                           ) &&
+                          (std::is_convertible<T, int>::value ||
+                           std::is_same<T, detail::byte>::value) &&
                           !has_formatter<T, Context>::value &&
                           !has_fallback_formatter<T, char_type>::value)>
   FMT_CONSTEXPR FMT_INLINE auto map(const T& val)