Remove deprecated fallback formatter

include/fmt/ostream.h

@@ -8,8 +8,8 @@
 #ifndef FMT_OSTREAM_H_
 #define FMT_OSTREAM_H_
 
-#include <fstream>
-#include <ostream>
+#include <fstream>  // std::filebuf
+
 #if defined(_WIN32) && defined(__GLIBCXX__)
 #  include <ext/stdio_filebuf.h>
 #  include <ext/stdio_sync_filebuf.h>
@@ -21,42 +21,8 @@
 
 FMT_BEGIN_NAMESPACE
 
-template <typename OutputIt, typename Char> class basic_printf_context;
-
 namespace detail {
 
-// Checks if T has a user-defined operator<<.
-template <typename T, typename Char, typename Enable = void>
-class is_streamable {
- private:
-  template <typename U>
-  static auto test(int)
-      -> bool_constant<sizeof(std::declval<std::basic_ostream<Char>&>()
-                              << std::declval<U>()) != 0>;
-
-  template <typename> static auto test(...) -> std::false_type;
-
-  using result = decltype(test<T>(0));
-
- public:
-  is_streamable() = default;
-
-  static const bool value = result::value;
-};
-
-// Formatting of built-in types and arrays is intentionally disabled because
-// it's handled by standard (non-ostream) formatters.
-template <typename T, typename Char>
-struct is_streamable<
-    T, Char,
-    enable_if_t<
-        std::is_arithmetic<T>::value || std::is_array<T>::value ||
-        std::is_pointer<T>::value || std::is_same<T, char8_type>::value ||
-        std::is_convertible<T, fmt::basic_string_view<Char>>::value ||
-        std::is_same<T, std_string_view<Char>>::value ||
-        (std::is_convertible<T, int>::value && !std::is_enum<T>::value)>>
-    : std::false_type {};
-
 // Generate a unique explicit instantion in every translation unit using a tag
 // type in an anonymous namespace.
 namespace {
@@ -180,13 +146,6 @@ auto streamed(const T& value) -> detail::streamed_view<T> {
 
 namespace detail {
 
-// Formats an object of type T that has an overloaded ostream operator<<.
-template <typename T, typename Char>
-struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
-    : basic_ostream_formatter<Char> {
-  using basic_ostream_formatter<Char>::format;
-};
-
 inline void vprint_directly(std::ostream& os, string_view format_str,
                             format_args args) {
   auto buffer = memory_buffer();