Reenable support for fallback formatter to join (fmtlib#2040)

include/fmt/format.h

@@ -3720,20 +3720,33 @@ struct arg_join : detail::view {
 };
 
 template <typename It, typename Sentinel, typename Char>
-struct formatter<arg_join<It, Sentinel, Char>, Char>
-    : formatter<typename std::iterator_traits<It>::value_type, Char> {
+struct formatter<arg_join<It, Sentinel, Char>, Char> {
+ private:
+  using value_type = typename std::iterator_traits<It>::value_type;
+  using formatter_type =
+      conditional_t<has_formatter<value_type, format_context>::value,
+                    formatter<value_type, Char>,
+                    detail::fallback_formatter<value_type, Char>>;
+
+  formatter_type value_formatter_;
+
+ public:
+  template <typename ParseContext>
+  FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
+    return value_formatter_.parse(ctx);
+  }
+
   template <typename FormatContext>
   auto format(const arg_join<It, Sentinel, Char>& value, FormatContext& ctx)
       -> decltype(ctx.out()) {
-    using base = formatter<typename std::iterator_traits<It>::value_type, Char>;
     auto it = value.begin;
     auto out = ctx.out();
     if (it != value.end) {
-      out = base::format(*it++, ctx);
+      out = value_formatter_.format(*it++, ctx);
       while (it != value.end) {
         out = detail::copy_str<Char>(value.sep.begin(), value.sep.end(), out);
         ctx.advance_to(out);
-        out = base::format(*it++, ctx);
+        out = value_formatter_.format(*it++, ctx);
       }
     }
     return out;

test/ostream-test.cc

@@ -188,6 +188,11 @@ TEST(OStreamTest, Join) {
   EXPECT_EQ("1, 2, 3", fmt::format("{}", fmt::join(v, v + 3, ", ")));
 }
 
+TEST(OStreamTest, JoinFallbackFormatter) {
+  auto strs = std::vector<TestString>{TestString("foo"), TestString("bar")};
+  EXPECT_EQ("foo, bar", fmt::format("{}", fmt::join(strs, ", ")));
+}
+
 #if FMT_USE_CONSTEXPR
 TEST(OStreamTest, ConstexprString) {
   EXPECT_EQ("42", format(FMT_STRING("{}"), std::string("42")));