Fix the returned value of `format_to_n` with user-defined types having operator<<.

include/fmt/ostream.h

@@ -124,8 +124,7 @@ struct formatter<T, Char,
     basic_memory_buffer<Char> buffer;
     internal::format_value(buffer, value);
     basic_string_view<Char> str(buffer.data(), buffer.size());
-    formatter<basic_string_view<Char>, Char>::format(str, ctx);
-    return ctx.out();
+    return formatter<basic_string_view<Char>, Char>::format(str, ctx);
   }
 };
 

test/ostream-test.cc

@@ -169,3 +169,25 @@ TEST(OStreamTest, ConstexprString) {
   EXPECT_EQ("42", format(fmt("{}"), std::string("42")));
 }
 #endif
+
+namespace fmt_test {
+struct ABC {};
+
+template <typename Output> Output &operator<<(Output &out, ABC) {
+  out << "ABC";
+  return out;
+}
+} // namespace fmt_test
+
+TEST(FormatTest, FormatToN) {
+  char buffer[4];
+  buffer[3] = 'x';
+  auto result = fmt::format_to_n(buffer, 3, "{}", fmt_test::ABC());
+  EXPECT_EQ(3u, result.size);
+  EXPECT_EQ(buffer + 3, result.out);
+  EXPECT_EQ("ABCx", fmt::string_view(buffer, 4));
+  result = fmt::format_to_n(buffer, 3, "x{}y", fmt_test::ABC());
+  EXPECT_EQ(5u, result.size);
+  EXPECT_EQ(buffer + 3, result.out);
+  EXPECT_EQ("xABx", fmt::string_view(buffer, 4));
+}