Fix handling of fixed enums in clang (#580)

include/fmt/core.h

@@ -82,6 +82,8 @@
 # endif
 #endif
 
+#define FMT_USE_STRONG_ENUMS FMT_HAS_FEATURE(cxx_strong_enums)
+
 // Check if exceptions are disabled.
 #if defined(__GNUC__) && !defined(__EXCEPTIONS)
 # define FMT_EXCEPTIONS 0
@@ -583,6 +585,12 @@ void make_value(const T *p) {
   static_assert(!sizeof(T), "formatting of non-void pointers is disallowed");
 }
 
+template <typename C, typename T>
+inline typename std::enable_if<
+    convert_to_int<T>::value && std::is_enum<T>::value,
+    typed_value<C, INT>>::type
+  make_value(const T &val) { return static_cast<int>(val); }
+
 template <typename C, typename T>
 inline typename std::enable_if<
     !convert_to_int<T>::value, typed_value<C, CUSTOM>>::type

test/format-test.cc

@@ -1508,6 +1508,14 @@ TEST(FormatTest, Enum) {
   EXPECT_EQ("0", fmt::format("{}", A));
 }
 
+#if FMT_USE_STRONG_ENUMS
+enum TestFixedEnum : short { B };
+
+TEST(FormatTest, FixedEnum) {
+  EXPECT_EQ("0", fmt::format("{}", B));
+}
+#endif
+
 using buffer_range = fmt::back_insert_range<fmt::internal::buffer>;
 
 class mock_arg_formatter :