Issue 3545

include/fmt/core.h

@@ -273,6 +273,17 @@ template <typename T> struct is_contiguous : std::false_type {};
 template <typename Char>
 struct is_contiguous<std::basic_string<Char>> : std::true_type {};
 
+template<typename E, typename = void>
+struct is_scoped_enum : std::false_type {};
+template<typename E>
+struct is_scoped_enum<E, std::enable_if_t<
+    std::is_enum<E>::value && 
+    !std::is_convertible<E, typename std::underlying_type<E>::type>::value 
+# ifdef __cpp_lib_byte
+    && !std::is_same<E, std::byte>::value
+# endif
+>> : std::true_type {};
+
 struct monostate {
   constexpr monostate() {}
 };

include/fmt/format.h

@@ -4290,8 +4290,13 @@ auto join(Range&& range, string_view sep)
 template <typename T, FMT_ENABLE_IF(!std::is_integral<T>::value)>
 inline auto to_string(const T& value) -> std::string {
   auto buffer = memory_buffer();
-  detail::write<char>(appender(buffer), value);
-  return {buffer.data(), buffer.size()};
+  if constexpr (is_scoped_enum<T>::value){
+    detail::write<char>(appender(buffer), static_cast<typename std::underlying_type<T>::type>(value));
+    return {buffer.data(), buffer.size()};
+  }else{
+    detail::write<char>(appender(buffer), value);
+    return {buffer.data(), buffer.size()};
+  }
 }
 
 template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>

test/format-test.cc

@@ -1944,11 +1944,15 @@ struct formatter<adl_test::fmt::detail::foo> : formatter<std::string> {
 };
 FMT_END_NAMESPACE
 
+enum class test_enum_class : char { one='1', two='2' };
+auto format_as(test_enum_class e) -> char { return static_cast<char>(e); }
+
 TEST(format_test, to_string) {
   EXPECT_EQ(fmt::to_string(42), "42");
   EXPECT_EQ(fmt::to_string(reinterpret_cast<void*>(0x1234)), "0x1234");
   EXPECT_EQ(fmt::to_string(adl_test::fmt::detail::foo()), "foo");
   EXPECT_EQ(fmt::to_string(foo), "0");
+  EXPECT_EQ(fmt::to_string(test_enum_class::one),"1");
 
 #if FMT_USE_FLOAT128
   EXPECT_EQ(fmt::to_string(__float128(0.5)), "0.5");