Add fmt::streamed

include/fmt/ostream.h

@@ -122,6 +122,9 @@ void format_value(buffer<Char>& buf, const T& value,
   output << value;
   output.exceptions(std::ios_base::failbit | std::ios_base::badbit);
 }
+
+template <typename T> struct streamed_view { const T& value; };
+
 }  // namespace detail
 
 // Formats an object of type T that has an overloaded ostream operator<<.
@@ -139,6 +142,20 @@ struct basic_ostream_formatter : formatter<basic_string_view<Char>, Char> {
 
 using ostream_formatter = basic_ostream_formatter<char>;
 
+template <typename T>
+struct formatter<detail::streamed_view<T>> : ostream_formatter {
+  template <typename OutputIt>
+  auto format(detail::streamed_view<T> view,
+              basic_format_context<OutputIt, char>& ctx) const -> OutputIt {
+    return ostream_formatter::format(view.value, ctx);
+  }
+};
+
+template <typename T>
+auto streamed(const T& value) -> detail::streamed_view<T> {
+  return {value};
+}
+
 namespace detail {
 
 // Formats an object of type T that has an overloaded ostream operator<<.
@@ -150,8 +167,7 @@ struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
 
 }  // namespace detail
 
-FMT_MODULE_EXPORT
-template <typename Char>
+FMT_MODULE_EXPORT template <typename Char>
 void vprint(std::basic_ostream<Char>& os,
             basic_string_view<type_identity_t<Char>> format_str,
             basic_format_args<buffer_context<type_identity_t<Char>>> args) {
@@ -169,8 +185,7 @@ void vprint(std::basic_ostream<Char>& os,
     fmt::print(cerr, "Don't {}!", "panic");
   \endrst
  */
-FMT_MODULE_EXPORT
-template <typename... T>
+FMT_MODULE_EXPORT template <typename... T>
 void print(std::ostream& os, format_string<T...> fmt, T&&... args) {
   vprint(os, fmt, fmt::make_format_args(args...));
 }

test/ostream-test.cc

@@ -32,12 +32,12 @@ template <> struct formatter<test> : formatter<int> {
 #include "gtest-extra.h"
 #include "util.h"
 
-std::ostream& operator<<(std::ostream& os, const date& d) {
+auto operator<<(std::ostream& os, const date& d) -> std::ostream& {
   os << d.year() << '-' << d.month() << '-' << d.day();
   return os;
 }
 
-std::wostream& operator<<(std::wostream& os, const date& d) {
+auto operator<<(std::wostream& os, const date& d) -> std::wostream& {
   os << d.year() << L'-' << d.month() << L'-' << d.day();
   return os;
 }
@@ -49,14 +49,16 @@ template <typename T> type_with_comma_op operator<<(T&, const date&);
 
 enum streamable_enum {};
 
-std::ostream& operator<<(std::ostream& os, streamable_enum) {
+auto operator<<(std::ostream& os, streamable_enum) -> std::ostream& {
   return os << "streamable_enum";
 }
 
 enum unstreamable_enum {};
 
 struct empty_test {};
-std::ostream& operator<<(std::ostream& os, empty_test) { return os << ""; }
+auto operator<<(std::ostream& os, empty_test) -> std::ostream& {
+  return os << "";
+}
 
 namespace fmt {
 template <> struct formatter<test_string> : ostream_formatter {};
@@ -130,7 +132,7 @@ TEST(ostream_test, write_to_ostream_max_size) {
 
   struct mock_streambuf : std::streambuf {
     MOCK_METHOD2(xsputn, std::streamsize(const void* s, std::streamsize n));
-    std::streamsize xsputn(const char* s, std::streamsize n) override {
+    auto xsputn(const char* s, std::streamsize n) -> std::streamsize override {
       const void* v = s;
       return xsputn(v, n);
     }
@@ -176,7 +178,7 @@ TEST(ostream_test, constexpr_string) {
 namespace fmt_test {
 struct abc {};
 
-template <typename Output> Output& operator<<(Output& out, abc) {
+template <typename Output> auto operator<<(Output& out, abc) -> Output& {
   return out << "abc";
 }
 }  // namespace fmt_test
@@ -184,7 +186,7 @@ template <typename Output> Output& operator<<(Output& out, abc) {
 template <typename T> struct test_template {};
 
 template <typename T>
-std::ostream& operator<<(std::ostream& os, test_template<T>) {
+auto operator<<(std::ostream& os, test_template<T>) -> std::ostream& {
   return os << 1;
 }
 
@@ -282,7 +284,7 @@ TEST(ostream_test, range) {
 struct abstract {
   virtual ~abstract() = default;
   virtual void f() = 0;
-  friend std::ostream& operator<<(std::ostream& os, const abstract&) {
+  friend auto operator<<(std::ostream& os, const abstract&) -> std::ostream& {
     return os;
   }
 };
@@ -300,6 +302,19 @@ TEST(ostream_test, is_formattable) {
   EXPECT_TRUE(fmt::is_formattable<fmt::detail::std_string_view<char>>());
 }
 
+struct streamable_and_unformattable {};
+
+auto operator<<(std::ostream& os, streamable_and_unformattable)
+    -> std::ostream& {
+  return os << "foo";
+}
+
+TEST(ostream_test, streamed) {
+  EXPECT_FALSE(fmt::is_formattable<streamable_and_unformattable>());
+  EXPECT_EQ(fmt::format("{}", fmt::streamed(streamable_and_unformattable())),
+            "foo");
+}
+
 TEST(ostream_test, closed_ofstream) {
   std::ofstream ofs;
   fmt::print(ofs, "discard");