Remove deprecated fallback formatter

include/fmt/core.h

@@ -1160,20 +1160,6 @@ auto get_iterator(buffer<T>&, OutputIt out) -> OutputIt {
   return out;
 }
 
-template <typename T, typename Char = char, typename Enable = void>
-struct fallback_formatter {
-  fallback_formatter() = delete;
-};
-
-// Specifies if T has an enabled fallback_formatter specialization.
-template <typename T, typename Char>
-using has_fallback_formatter =
-#ifdef FMT_DEPRECATED_OSTREAM
-    std::is_constructible<fallback_formatter<T, Char>>;
-#else
-    std::false_type;
-#endif
-
 struct view {};
 
 template <typename Char, typename T> struct named_arg : view {
@@ -1332,10 +1318,7 @@ template <typename Context> class value {
     // have different extension points, e.g. `formatter<T>` for `format` and
     // `printf_formatter<T>` for `printf`.
     custom.format = format_custom_arg<
-        value_type,
-        conditional_t<has_formatter<value_type, Context>::value,
-                      typename Context::template formatter_type<value_type>,
-                      fallback_formatter<value_type, char_type>>>;
+        value_type, typename Context::template formatter_type<value_type>>;
   }
   value(unformattable);
   value(unformattable_char);
@@ -1493,8 +1476,7 @@ template <typename Context> struct arg_mapper {
   template <typename T, typename U = remove_cvref_t<T>>
   struct formattable
       : bool_constant<has_const_formatter<U, Context>() ||
-                      !std::is_const<remove_reference_t<T>>::value ||
-                      has_fallback_formatter<U, char_type>::value> {};
+                      !std::is_const<remove_reference_t<T>>::value> {};
 
 #if (FMT_MSC_VERSION != 0 && FMT_MSC_VERSION < 1910) || \
     FMT_ICC_VERSION != 0 || defined(__NVCC__)
@@ -1518,8 +1500,7 @@ template <typename Context> struct arg_mapper {
                           !std::is_array<U>::value &&
                           !std::is_pointer<U>::value &&
                           !std::is_arithmetic<format_as_t<U>>::value &&
-                          (has_formatter<U, Context>::value ||
-                           has_fallback_formatter<U, char_type>::value))>
+                          has_formatter<U, Context>::value)>
   FMT_CONSTEXPR FMT_INLINE auto map(T&& val)
       -> decltype(this->do_map(std::forward<T>(val))) {
     return do_map(std::forward<T>(val));
@@ -1770,9 +1751,9 @@ FMT_CONSTEXPR auto make_arg(T&& value) -> basic_format_arg<Context> {
   return arg;
 }
 
-// The type template parameter is there to avoid an ODR violation when using
-// a fallback formatter in one translation unit and an implicit conversion in
-// another (not recommended).
+// The DEPRECATED type template parameter is there to avoid an ODR violation
+// when using a fallback formatter in one translation unit and an implicit
+// conversion in another (not recommended).
 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> {
@@ -1843,11 +1824,9 @@ using buffer_context =
 using format_context = buffer_context<char>;
 
 template <typename T, typename Char = char>
-using is_formattable = bool_constant<
-    !std::is_base_of<detail::unformattable,
-                     decltype(detail::arg_mapper<buffer_context<Char>>().map(
-                         std::declval<T>()))>::value &&
-    !detail::has_fallback_formatter<T, Char>::value>;
+using is_formattable = bool_constant<!std::is_base_of<
+    detail::unformattable, decltype(detail::arg_mapper<buffer_context<Char>>()
+                                        .map(std::declval<T>()))>::value>;
 
 /**
   \rst
@@ -2631,10 +2610,7 @@ FMT_CONSTEXPR auto parse_format_specs(ParseContext& ctx)
       mapped_type_constant<T, context>::value != type::custom_type,
       decltype(arg_mapper<context>().map(std::declval<const T&>())),
       stripped_type>;
-  auto f = conditional_t<has_formatter<mapped_type, context>::value,
-                         formatter<mapped_type, char_type>,
-                         fallback_formatter<stripped_type, char_type>>();
-  return f.parse(ctx);
+  return formatter<mapped_type, char_type>().parse(ctx);
 }
 
 // Checks char specs and returns true iff the presentation type is char-like.

include/fmt/format.h

@@ -3891,12 +3891,8 @@ template <typename Char, typename OutputIt, typename T,
 FMT_CONSTEXPR auto write(OutputIt out, const T& value)
     -> enable_if_t<mapped_type_constant<T, Context>::value == type::custom_type,
                    OutputIt> {
-  using formatter_type =
-      conditional_t<has_formatter<T, Context>::value,
-                    typename Context::template formatter_type<T>,
-                    fallback_formatter<T, Char>>;
   auto ctx = Context(out, {}, {});
-  return formatter_type().format(value, ctx);
+  return typename Context::template formatter_type<T>().format(value, ctx);
 }
 
 // An argument visitor that formats the argument and writes it via the output
@@ -4382,12 +4378,7 @@ struct formatter<join_view<It, Sentinel, Char>, Char> {
 #else
       typename std::iterator_traits<It>::value_type;
 #endif
-  using formatter_type =
-      conditional_t<is_formattable<value_type, Char>::value,
-                    formatter<remove_cvref_t<value_type>, Char>,
-                    detail::fallback_formatter<value_type, Char>>;
-
-  formatter_type value_formatter_;
+  formatter<remove_cvref_t<value_type>, Char> value_formatter_;
 
  public:
   template <typename ParseContext>

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();

include/fmt/ranges.h

@@ -404,12 +404,10 @@ template <typename Context> struct range_mapper {
 };
 
 template <typename Char, typename Element>
-using range_formatter_type = conditional_t<
-    is_formattable<Element, Char>::value,
+using range_formatter_type =
     formatter<remove_cvref_t<decltype(range_mapper<buffer_context<Char>>{}.map(
                   std::declval<Element>()))>,
-              Char>,
-    fallback_formatter<Element, Char>>;
+              Char>;
 
 template <typename R>
 using maybe_const_range =
@@ -419,9 +417,7 @@ using maybe_const_range =
 #if !FMT_MSC_VERSION || FMT_MSC_VERSION >= 1910
 template <typename R, typename Char>
 struct is_formattable_delayed
-    : disjunction<
-          is_formattable<uncvref_type<maybe_const_range<R>>, Char>,
-          has_fallback_formatter<uncvref_type<maybe_const_range<R>>, Char>> {};
+    : is_formattable<uncvref_type<maybe_const_range<R>>, Char> {};
 #endif
 }  // namespace detail
 
@@ -431,10 +427,8 @@ struct range_formatter;
 template <typename T, typename Char>
 struct range_formatter<
     T, Char,
-    enable_if_t<conjunction<
-        std::is_same<T, remove_cvref_t<T>>,
-        disjunction<is_formattable<T, Char>,
-                    detail::has_fallback_formatter<T, Char>>>::value>> {
+    enable_if_t<conjunction<std::is_same<T, remove_cvref_t<T>>,
+                            is_formattable<T, Char>>::value>> {
  private:
   detail::range_formatter_type<Char, T> underlying_;
   basic_string_view<Char> separator_ = detail::string_literal<Char, ',', ' '>{};