From 835b910e7d758efdfdce9f23df1b190deb3373db Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Sun, 28 Feb 2021 15:25:33 -0800
Subject: [PATCH] Add an is_formattable trait

---
 include/fmt/core.h | 5 +++++
 test/core-test.cc  | 6 ++++++
 2 files changed, 11 insertions(+)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index ec729116..94b31f71 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -1525,6 +1525,11 @@ using wformat_context = buffer_context<wchar_t>;
 #define FMT_BUFFER_CONTEXT(Char) \
   basic_format_context<detail::buffer_appender<Char>, Char>
 
+template <typename T, typename Char = char>
+using is_formattable = bool_constant<!std::is_same<
+    decltype(detail::arg_mapper<buffer_context<Char>>().map(std::declval<T>())),
+    detail::unformattable>::value>;
+
 /**
   \rst
   An array of references to arguments. It can be implicitly converted into
diff --git a/test/core-test.cc b/test/core-test.cc
index 50b5f9b8..5da58a60 100644
--- a/test/core-test.cc
+++ b/test/core-test.cc
@@ -617,6 +617,12 @@ TEST(CoreTest, HasFormatter) {
                 "");
 }
 
+TEST(CoreTest, IsFormattable) {
+  static_assert(fmt::is_formattable<enabled_formatter>::value, "");
+  static_assert(!fmt::is_formattable<disabled_formatter>::value, "");
+  static_assert(fmt::is_formattable<disabled_formatter_convertible>::value, "");
+}
+
 struct convertible_to_int {
   operator int() const { return 42; }
 };