From 31e743d06e739405eab934d30ba73461244ae13e Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Fri, 4 Feb 2022 11:00:00 -0800
Subject: [PATCH] Don't use ostream for types convertible to string_view

---
 include/fmt/core.h    |  2 +-
 include/fmt/ostream.h |  2 +-
 test/ostream-test.cc  | 15 +++++++++++++++
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 68b86852..f64f1a69 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -9,7 +9,7 @@
 #define FMT_CORE_H_
 
 #include <cstdio>   // std::FILE
-#include <cstring>
+#include <cstring>  // std::strlen
 #include <iterator>
 #include <limits>
 #include <string>
diff --git a/include/fmt/ostream.h b/include/fmt/ostream.h
index 5a2a5456..7acc3299 100644
--- a/include/fmt/ostream.h
+++ b/include/fmt/ostream.h
@@ -45,7 +45,7 @@ struct is_streamable<
     enable_if_t<
         std::is_arithmetic<T>::value || std::is_array<T>::value ||
         std::is_pointer<T>::value || std::is_same<T, char8_type>::value ||
-        std::is_same<T, std::basic_string<Char>>::value ||
+        std::is_convertible<T, fmt::basic_string_view<Char>>::value ||
         std::is_same<T, std_string_view<Char>>::value ||
         (std::is_convertible<T, int>::value && !std::is_enum<T>::value)>>
     : std::false_type {};
diff --git a/test/ostream-test.cc b/test/ostream-test.cc
index f81039e5..bae7ad18 100644
--- a/test/ostream-test.cc
+++ b/test/ostream-test.cc
@@ -262,6 +262,21 @@ TEST(ostream_test, format_convertible_to_bool) {
   EXPECT_EQ(fmt::format("{}", streamable_and_convertible_to_bool()), "true");
 }
 
+struct streamable_and_convertible_to_string_view {
+  operator fmt::string_view() const { return "foo"; }
+};
+
+std::ostream& operator<<(std::ostream& os,
+                         streamable_and_convertible_to_string_view) {
+  return os << "bar";
+}
+
+TEST(ostream_test, format_convertible_to_string_vew) {
+  // operator<< is intentionally not used because of potential ODR violations.
+  EXPECT_EQ(fmt::format("{}", streamable_and_convertible_to_string_view()),
+            "foo");
+}
+
 struct copyfmt_test {};
 
 std::ostream& operator<<(std::ostream& os, copyfmt_test) {