Revert "Replace a bunch of craft with type_traits"

This reverts commit e79588d6c1.

include/fmt/ostream.h

@@ -47,12 +47,22 @@ class FormatBuf : public std::basic_streambuf<Char> {
   }
 };
 
-template <typename T>
-struct convert_to_int<T,
-    sizeof(std::declval<std::ostream>() << std::declval<T>()) != 0> {
+yes &convert(std::ostream &);
+
+struct DummyStream : std::ostream {
+  DummyStream();  // Suppress a bogus warning in MSVC.
+  // Hide all operator<< overloads from std::ostream.
+  void operator<<(null<>);
+};
+
+no &operator<<(std::ostream &, int);
+
+template<typename T>
+struct convert_to_int_impl<T, true> {
   // Convert to int only if T doesn't have an overloaded operator<<.
   enum {
-    value = false
+    value = sizeof(convert(std::declval<DummyStream&>() << std::declval<T>()))
+            == sizeof(no)
   };
 };