From 17b362f78ce5e1322c13e7e19b30e03e88dc62c1 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Fri, 4 Feb 2022 18:33:55 -0800
Subject: [PATCH] Simplify ostream opt-in API

---
 include/fmt/ostream.h | 23 ++++++++++++++---------
 test/ostream-test.cc  | 16 +++++++---------
 test/xchar-test.cc    |  4 ++--
 3 files changed, 23 insertions(+), 20 deletions(-)

diff --git a/include/fmt/ostream.h b/include/fmt/ostream.h
index 4634bcf9..2bb79bbb 100644
--- a/include/fmt/ostream.h
+++ b/include/fmt/ostream.h
@@ -78,14 +78,12 @@ void format_value(buffer<Char>& buf, const T& value,
   output.exceptions(std::ios_base::failbit | std::ios_base::badbit);
   buf.try_resize(buf.size());
 }
+}  // namespace detail
 
 // Formats an object of type T that has an overloaded ostream operator<<.
-template <typename T, typename Char>
-struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
-    : private formatter<basic_string_view<Char>, Char> {
-  using formatter<basic_string_view<Char>, Char>::parse;
-
-  template <typename OutputIt>
+template <typename Char>
+struct basic_ostream_formatter : formatter<basic_string_view<Char>, Char> {
+  template <typename T, typename OutputIt>
   auto format(const T& value, basic_format_context<OutputIt, Char>& ctx) const
       -> OutputIt {
     auto buffer = basic_memory_buffer<Char>();
@@ -93,7 +91,17 @@ struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
     return formatter<basic_string_view<Char>, Char>::format(
         {buffer.data(), buffer.size()}, ctx);
   }
+};
 
+using ostream_formatter = basic_ostream_formatter<char>;
+
+namespace detail {
+
+// Formats an object of type T that has an overloaded ostream operator<<.
+template <typename T, typename Char>
+struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
+    : basic_ostream_formatter<Char> {
+  using basic_ostream_formatter<Char>::format;
   // DEPRECATED!
   template <typename OutputIt>
   auto format(const T& value, basic_printf_context<OutputIt, Char>& ctx) const
@@ -105,9 +113,6 @@ struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
 };
 }  // namespace detail
 
-template <typename T, typename Char = char>
-using ostream_formatter = detail::fallback_formatter<T, Char>;
-
 FMT_MODULE_EXPORT
 template <typename Char>
 void vprint(std::basic_ostream<Char>& os, basic_string_view<Char> format_str,
diff --git a/test/ostream-test.cc b/test/ostream-test.cc
index 2f9c4383..3834c707 100644
--- a/test/ostream-test.cc
+++ b/test/ostream-test.cc
@@ -57,11 +57,10 @@ struct empty_test {};
 std::ostream& operator<<(std::ostream& os, empty_test) { return os << ""; }
 
 namespace fmt {
-template <> struct formatter<test_string> : ostream_formatter<test_string> {};
-template <> struct formatter<date> : ostream_formatter<date> {};
-template <>
-struct formatter<streamable_enum> : ostream_formatter<streamable_enum> {};
-template <> struct formatter<empty_test> : ostream_formatter<empty_test> {};
+template <> struct formatter<test_string> : ostream_formatter {};
+template <> struct formatter<date> : ostream_formatter {};
+template <> struct formatter<streamable_enum> : ostream_formatter {};
+template <> struct formatter<empty_test> : ostream_formatter {};
 }  // namespace fmt
 
 TEST(ostream_test, enum) {
@@ -193,8 +192,7 @@ template <typename T> struct formatter<test_template<T>> : formatter<int> {
   }
 };
 
-template <>
-struct formatter<fmt_test::abc> : ostream_formatter<fmt_test::abc> {};
+template <> struct formatter<fmt_test::abc> : ostream_formatter {};
 }  // namespace fmt
 
 TEST(ostream_test, template) {
@@ -262,7 +260,7 @@ std::ostream& operator<<(std::ostream& os, copyfmt_test) {
 }
 
 namespace fmt {
-template <> struct formatter<copyfmt_test> : ostream_formatter<copyfmt_test> {};
+template <> struct formatter<copyfmt_test> : ostream_formatter {};
 }  // namespace fmt
 
 TEST(ostream_test, copyfmt) {
@@ -287,7 +285,7 @@ struct abstract {
 };
 
 namespace fmt {
-template <> struct formatter<abstract> : ostream_formatter<abstract> {};
+template <> struct formatter<abstract> : ostream_formatter {};
 }  // namespace fmt
 
 void format_abstract_compiles(const abstract& a) {
diff --git a/test/xchar-test.cc b/test/xchar-test.cc
index 2d7dfa85..40d7fca1 100644
--- a/test/xchar-test.cc
+++ b/test/xchar-test.cc
@@ -222,8 +222,8 @@ std::wostream& operator<<(std::wostream& os, streamable_enum) {
 
 namespace fmt {
 template <>
-struct formatter<streamable_enum, wchar_t>
-    : ostream_formatter<streamable_enum, wchar_t> {};
+struct formatter<streamable_enum, wchar_t> : basic_ostream_formatter<wchar_t> {
+};
 }  // namespace fmt
 
 enum unstreamable_enum {};