Backport fmt PR #1829

Summary: Backport fmtlib/fmt#1829 to make sure that `fmt::to_string` supports streamable types.

Reviewed By: h-friederich

Differential Revision: D23387021

fbshipit-source-id: f538525a6fe2a296b508164f19a8bc0704183666

deps/fmt/include/fmt/format.h

@@ -1810,8 +1810,13 @@ 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

deps/fmt/test/ostream-test.cc

@@ -307,3 +307,7 @@ 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()));
+}