Fix detail::write with fallback formatter (#1829)

* add support for fallback_formatter in detail::write

* add ToString test into OStreamTest

to check fmt::to_string() with class that has output stream operator

* add WithOstreamOperator test into CompileTest

to check fmt::format() with FMT_COMPILE() and class that has output stream operator

* use conditional_t inside detail::write instead of 2 overloads

* Revert "add WithOstreamOperator test into CompileTest"

* remove Context from template parameters in detail::write

include/fmt/format.h

@@ -1847,8 +1847,13 @@ auto write(OutputIt out, const T& value) -> typename std::enable_if<
     mapped_type_constant<T, basic_format_context<OutputIt, Char>>::value ==
         type::custom_type,
     OutputIt>::type {
-  basic_format_context<OutputIt, Char> ctx(out, {}, {});
-  return formatter<T>().format(value, ctx);
+  using context_type = basic_format_context<OutputIt, Char>;
+  using formatter_type =
+      conditional_t<has_formatter<T, context_type>::value,
+                    typename context_type::template formatter_type<T>,
+                    fallback_formatter<T, Char>>;
+  context_type ctx(out, {}, {});
+  return formatter_type().format(value, ctx);
 }
 
 // An argument visitor that formats the argument and writes it via the output

test/ostream-test.cc

@@ -319,3 +319,7 @@ TEST(OStreamTest, CopyFmt) {
 TEST(OStreamTest, CompileTimeString) {
   EXPECT_EQ("42", fmt::format(FMT_STRING("{}"), 42));
 }
+
+TEST(OStreamTest, ToString) {
+  EXPECT_EQ("ABC", fmt::to_string(fmt_test::ABC()));
+}