Add an is_formattable trait

Author: vitaut

include/fmt/core.h

@@ -1525,6 +1525,11 @@ using wformat_context = buffer_context<wchar_t>;
 #define FMT_BUFFER_CONTEXT(Char) \
   basic_format_context<detail::buffer_appender<Char>, Char>
 
+template <typename T, typename Char = char>
+using is_formattable = bool_constant<!std::is_same<
+    decltype(detail::arg_mapper<buffer_context<Char>>().map(std::declval<T>())),
+    detail::unformattable>::value>;
+
 /**
   \rst
   An array of references to arguments. It can be implicitly converted into

test/core-test.cc

@@ -617,6 +617,12 @@ TEST(CoreTest, HasFormatter) {
                 "");
 }
 
+TEST(CoreTest, IsFormattable) {
+  static_assert(fmt::is_formattable<enabled_formatter>::value, "");
+  static_assert(!fmt::is_formattable<disabled_formatter>::value, "");
+  static_assert(fmt::is_formattable<disabled_formatter_convertible>::value, "");
+}
+
 struct convertible_to_int {
   operator int() const { return 42; }
 };