Improve comments

include/fmt/core.h

@@ -1222,6 +1222,8 @@ using long_type = conditional_t<long_short, int, long long>;
 using ulong_type = conditional_t<long_short, unsigned, unsigned long long>;
 
 // Maps formatting arguments to core types.
+// arg_mapper reports errors by returning unformattable instead of using
+// static_assert because it's used in the is_formattable trait.
 template <typename Context> struct arg_mapper {
   using char_type = typename Context::char_type;
 
@@ -1325,10 +1327,6 @@ template <typename Context> struct arg_mapper {
   template <typename T>
   FMT_CONSTEXPR auto map(T)
       -> enable_if_t<std::is_pointer<T>::value, unformattable_pointer> {
-    // Formatting of arbitrary pointers is disallowed. If you want to output
-    // a pointer cast it to "void *" or "const void *". In particular, this
-    // forbids formatting of "[const] volatile char *" which is printed as bool
-    // by iostreams.
     return {};
   }
 
@@ -1593,10 +1591,14 @@ template <bool IS_PACKED, typename Context, type, typename T,
           FMT_ENABLE_IF(IS_PACKED)>
 FMT_CONSTEXPR FMT_INLINE auto make_arg(T&& val) -> value<Context> {
   const auto& arg = arg_mapper<Context>().map(std::forward<T>(val));
+
+  // Formatting of arbitrary pointers is disallowed. If you want to output
+  // a pointer cast it to "void *" or "const void *". In particular, this
+  // forbids formatting of "[const] volatile char *" which is printed as bool
+  // by iostreams.
   constexpr bool void_ptr =
       !std::is_same<decltype(arg), const unformattable_pointer&>::value;
-  static_assert(void_ptr,
+  static_assert(void_ptr, "Formatting of non-void pointers is disallowed.");
-                "Formatting of non-void pointers is disallowed.");
   constexpr bool formattable =
       !std::is_same<decltype(arg), const unformattable&>::value;
   static_assert(