Suspicious logic in TypeSymbolExtensions.IsValidEnumType

[AlekseyTs]

        public static bool IsValidEnumType(this TypeSymbol type)
        {
            var underlyingType = type.GetEnumUnderlyingType();
            // SpecialType will be None if the underlying type is invalid.
            return (underlyingType is object) && (underlyingType.SpecialType != SpecialType.None);
        }

The underlyingType.SpecialType != SpecialType.None logic looks questionable, in fact we have a SpecialType extension called IsValidEnumUnderlyingType, which we probably should use instead.