From 8d545d7d6b7d45ec362309efbaf6c0549ec1bbba Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Fri, 28 Aug 2020 03:28:50 -0700
Subject: [PATCH] Backport fmt PR #1829

Summary: Backport https://github.com/fmtlib/fmt/pull/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 | 9 +++++++--
 deps/fmt/test/ostream-test.cc | 4 ++++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/deps/fmt/include/fmt/format.h b/deps/fmt/include/fmt/format.h
index 29221289..20ec7ee1 100644
--- a/deps/fmt/include/fmt/format.h
+++ b/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
diff --git a/deps/fmt/test/ostream-test.cc b/deps/fmt/test/ostream-test.cc
index 1c87d46d..336c1ea5 100644
--- a/deps/fmt/test/ostream-test.cc
+++ b/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()));
+}