Make formatter<T> override ostream<< for templates (#952)

include/fmt/core.h

@@ -331,9 +331,6 @@ struct error_handler {
   FMT_API void on_error(const char* message);
 };
 
-template <typename T> struct no_formatter_error : std::false_type {};
-}  // namespace internal
-
 // GCC 4.6.x cannot expand `T...`.
 #if FMT_GCC_VERSION && FMT_GCC_VERSION < 407
 template <typename... T> struct is_constructible : std::false_type {};
@@ -342,6 +339,16 @@ template <typename... T>
 struct is_constructible : std::is_constructible<T...> {};
 #endif
 
+typedef char yes[1];
+typedef char no[2];
+
+template <typename T> struct is_default_constructible {
+  template <typename U> static yes& test(int (*)[sizeof(new U)]);
+  template <typename U> static no& test(...);
+  enum { value = sizeof(test<T>(0)) == sizeof(yes) };
+};
+}  // namespace internal
+
 /**
   An implementation of ``std::basic_string_view`` for pre-C++17. It provides a
   subset of the API. ``fmt::basic_string_view`` is used for format strings even
@@ -492,22 +499,12 @@ FMT_CONSTEXPR basic_string_view<typename S::char_type> to_string_view(
 }
 
 template <typename Context> class basic_format_arg;
-
 template <typename Context> class basic_format_args;
 
 // A formatter for objects of type T.
 template <typename T, typename Char = char, typename Enable = void>
 struct formatter {
-  static_assert(
-      internal::no_formatter_error<T>::value,
-      "don't know how to format the type, include fmt/ostream.h if it provides "
-      "an operator<< that should be used");
-
-  // The following functions are not defined intentionally.
-  template <typename ParseContext>
-  typename ParseContext::iterator parse(ParseContext&);
-  template <typename FormatContext>
-  auto format(const T& val, FormatContext& ctx) -> decltype(ctx.out());
+  formatter() = delete;
 };
 
 template <typename T, typename Char, typename Enable = void>
@@ -517,6 +514,16 @@ struct convert_to_int
 
 namespace internal {
 
+template <typename T> struct no_formatter_error : std::false_type {};
+
+template <typename T, typename Char = char, typename Enable = void>
+struct fallback_formatter {
+  static_assert(
+      no_formatter_error<T>::value,
+      "don't know how to format the type, include fmt/ostream.h if it provides "
+      "an operator<< that should be used");
+};
+
 struct dummy_string_view {
   typedef void char_type;
 };
@@ -623,9 +630,29 @@ template <typename Context> class value {
   }
   value(const void* val) { pointer = val; }
 
-  template <typename T> explicit value(const T& val) {
+  template <typename T,
+            typename std::enable_if<
+                is_default_constructible<
+                    typename Context::template formatter_type<T>::type>::value,
+                int>::type = 0>
+  explicit value(const T& val) {
+    custom.value = &val;
+    // Get the formatter type through the context to allow different contexts
+    // have different extension points, e.g. `formatter<T>` for `format` and
+    // `printf_formatter<T>` for `printf`.
+    typedef typename Context::template formatter_type<T>::type formatter;
+    custom.format = &format_custom_arg<T, formatter>;
+  }
+
+  template <typename T,
+            typename std::enable_if<
+                !is_default_constructible<
+                    typename Context::template formatter_type<T>::type>::value,
+                int>::type = 0>
+  explicit value(const T& val) {
     custom.value = &val;
-    custom.format = &format_custom_arg<T>;
+    custom.format =
+        &format_custom_arg<T, internal::fallback_formatter<T, char_type>>;
   }
 
   const named_arg_base<char_type>& as_named_arg() {
@@ -634,12 +661,9 @@ template <typename Context> class value {
 
  private:
   // Formats an argument of a custom type, such as a user-defined class.
-  template <typename T>
+  template <typename T, typename Formatter>
   static void format_custom_arg(const void* arg, Context& ctx) {
-    // Get the formatter type through the context to allow different contexts
-    // have different extension points, e.g. `formatter<T>` for `format` and
-    // `printf_formatter<T>` for `printf`.
-    typename Context::template formatter_type<T>::type f;
+    Formatter f;
     auto&& parse_ctx = ctx.parse_context();
     parse_ctx.advance_to(f.parse(parse_ctx));
     ctx.advance_to(f.format(*static_cast<const T*>(arg), ctx));

include/fmt/format.h

@@ -1702,8 +1702,7 @@ class specs_handler : public specs_setter<typename Context::char_type> {
 
   FMT_CONSTEXPR format_arg get_arg(auto_id) { return context_.next_arg(); }
 
-  template <typename Id>
-  FMT_CONSTEXPR format_arg get_arg(Id arg_id) {
+  template <typename Id> FMT_CONSTEXPR format_arg get_arg(Id arg_id) {
     context_.parse_context().check_arg_id(arg_id);
     return context_.arg(arg_id);
   }

include/fmt/ostream.h

@@ -93,22 +93,12 @@ void format_value(basic_buffer<Char>& buffer, const T& value) {
   output << value;
   buffer.resize(buffer.size());
 }
-}  // namespace internal
-
-// Disable conversion to int if T has an overloaded operator<< which is a free
-// function (not a member of std::ostream).
-template <typename T, typename Char> struct convert_to_int<T, Char, void> {
-  static const bool value = convert_to_int<T, Char, int>::value &&
-                            !internal::is_streamable<T, Char>::value;
-};
 
 // Formats an object of type T that has an overloaded ostream operator<<.
 template <typename T, typename Char>
-struct formatter<T, Char,
-                 typename std::enable_if<
-                     internal::is_streamable<T, Char>::value &&
-                     !internal::format_type<typename buffer_context<Char>::type,
-                                            T>::value>::type>
+struct fallback_formatter<
+    T, Char,
+    typename std::enable_if<internal::is_streamable<T, Char>::value>::type>
     : formatter<basic_string_view<Char>, Char> {
   template <typename Context>
   auto format(const T& value, Context& ctx) -> decltype(ctx.out()) {
@@ -118,6 +108,14 @@ struct formatter<T, Char,
     return formatter<basic_string_view<Char>, Char>::format(str, ctx);
   }
 };
+}  // namespace internal
+
+// Disable conversion to int if T has an overloaded operator<< which is a free
+// function (not a member of std::ostream).
+template <typename T, typename Char> struct convert_to_int<T, Char, void> {
+  static const bool value = convert_to_int<T, Char, int>::value &&
+                            !internal::is_streamable<T, Char>::value;
+};
 
 template <typename Char>
 inline void vprint(

test/format-test.cc

@@ -2144,6 +2144,10 @@ struct test_context {
   typedef char char_type;
   typedef fmt::basic_format_arg<test_context> format_arg;
 
+  template <typename T> struct formatter_type {
+    typedef fmt::formatter<T, char_type> type;
+  };
+
   FMT_CONSTEXPR fmt::basic_format_arg<test_context> next_arg() {
     return fmt::internal::make_arg<test_context>(11);
   }

test/ostream-test.cc

@@ -178,6 +178,29 @@ template <typename Output> Output& operator<<(Output& out, ABC) {
 }
 }  // namespace fmt_test
 
+template <typename T>
+struct TestTemplate {};
+
+template <typename T>
+std::ostream& operator<<(std::ostream& os, TestTemplate<T>) {
+  return os << 1;
+}
+
+namespace fmt {
+template <typename T>
+struct formatter<TestTemplate<T>> : formatter<int> {
+  template <typename FormatContext>
+  typename FormatContext::iterator format(TestTemplate<T>, FormatContext& ctx) {
+    return formatter<int>::format(2, ctx);
+  }
+};
+}
+
+#if !FMT_GCC_VERSION || FMT_GCC_VERSION >= 407
+TEST(OStreamTest, Template) {
+  EXPECT_EQ("2", fmt::format("{}", TestTemplate<int>()));
+}
+
 TEST(FormatTest, FormatToN) {
   char buffer[4];
   buffer[3] = 'x';
@@ -190,6 +213,7 @@ TEST(FormatTest, FormatToN) {
   EXPECT_EQ(buffer + 3, result.out);
   EXPECT_EQ("xABx", fmt::string_view(buffer, 4));
 }
+#endif
 
 #if FMT_USE_USER_DEFINED_LITERALS
 TEST(FormatTest, UDL) {