Fix handling of user-defined types in format_to (#793)

include/fmt/format.h

@@ -3216,9 +3216,8 @@ struct formatter<
       specs_.precision_, specs_.precision_ref, ctx);
     typedef output_range<typename FormatContext::iterator,
                          typename FormatContext::char_type> range_type;
-    visit(arg_formatter<range_type>(ctx, specs_),
+    return visit(arg_formatter<range_type>(ctx, specs_),
                  internal::make_arg<FormatContext>(val));
-    return ctx.out();
   }
 
  private:

test/format-test.cc

@@ -1122,7 +1122,8 @@ class Answer {};
 FMT_BEGIN_NAMESPACE
 template <>
 struct formatter<Answer> : formatter<int> {
-  auto format(Answer, fmt::format_context &ctx) -> decltype(ctx.out()) {
+  template <typename FormatContext>
+  auto format(Answer, FormatContext &ctx) -> decltype(ctx.out()) {
     return formatter<int>::format(42, ctx);
   }
 };
@@ -1133,6 +1134,13 @@ TEST(FormatterTest, CustomFormat) {
   EXPECT_EQ("0042", format("{:04}", Answer()));
 }
 
+TEST(FormatterTest, CustomFormatTo) {
+  char buf[10] = {};
+  auto end = fmt::format_to(buf, "{}", Answer());
+  EXPECT_EQ(end, buf + 2);
+  EXPECT_STREQ(buf, "42");
+}
+
 TEST(FormatterTest, WideFormatString) {
   EXPECT_EQ(L"42", format(L"{}", 42));
   EXPECT_EQ(L"4.2", format(L"{}", 4.2));