Fix handling of fixed enums in clang (#580)

fmt/format.h

@@ -1405,6 +1405,20 @@ class MakeValue : public Arg {
   FMT_MAKE_VALUE(unsigned char, uint_value, UINT)
   FMT_MAKE_VALUE(char, int_value, CHAR)
 
+#if __cplusplus >= 201103L
+  template <
+    typename T,
+    typename = typename std::enable_if<
+      std::is_enum<T>::value && ConvertToInt<T>::value>::type>
+   MakeValue(T value) { int_value = value; }
+
+  template <
+    typename T,
+    typename = typename std::enable_if<
+      std::is_enum<T>::value && ConvertToInt<T>::value>::type>
+  static uint64_t type(T value) { return Arg::INT; }
+#endif
+
 #if !defined(_MSC_VER) || defined(_NATIVE_WCHAR_T_DEFINED)
   MakeValue(typename WCharHelper<wchar_t, Char>::Supported value) {
     int_value = value;

test/format-test.cc

@@ -1681,6 +1681,14 @@ TEST(FormatTest, Enum) {
   EXPECT_EQ("0", fmt::format("{}", A));
 }
 
+#if __cplusplus >= 201103L
+enum TestFixedEnum : short { B };
+
+TEST(FormatTest, FixedEnum) {
+  EXPECT_EQ("0", fmt::format("{}", B));
+}
+#endif
+
 class MockArgFormatter :
     public fmt::internal::ArgFormatterBase<MockArgFormatter, char> {
  public: