如何将enable-if与模板参数和参数包一起使用

我在处理c++模板时，发现了这段使用SFINAE和std::enable_if的代码。我在使用此代码时遇到两个问题。

#include <string>
#include <iostream>
enum class Type : int { First = 1, Second, Third };
template <Type T>
struct Symbol : public std::true_type {
using Parent = std::true_type;
using type = Parent::value_type;
static constexpr type value = Parent::value;
static constexpr type GetValue() {
if constexpr (T == Type::First) return value;
else return !value;
}
};

template <bool b>
using EnableIf = typename std::enable_if<b, int>::type;
template <Type T> static constexpr bool IsTradingSymbol() {
return Symbol<T>::GetValue();
}
template <Type T, EnableIf<IsTradingSymbol<T>()>...>
bool GetErrorForUi(Type A) {
return true;
}
template <Type T, EnableIf<!IsTradingSymbol<T>()>...>
bool GetErrorForUi(Type A) {
return false;
}
int main () {
std::cout << GetErrorForUi<Type::First>(Type::First) << std::endl;
std::cout << GetErrorForUi<Type::Second>(Type::Second) << std::endl;
return 0;
}

1(EnableIf<IsTradingSymbol<T>()>...中为什么使用参数包。如果我删除...，那么代码在编译中失败。我无法从跟踪中推断出确切的错误，为什么需要这个...？(根据GCC 7.4.0编制(。

enableif.cpp: In function ‘int main()’:
enableif.cpp:43:54: error: no matching function for call to ‘GetErrorForUi<First>(Type)’
std::cout << GetErrorForUi<Type::First>(Type::First) << std::endl;
^
enableif.cpp:26:6: note: candidate: template<Type T, typename std::enable_if<IsTradingSymbol<T>(), int>::type <anonymous> > bool GetErrorForUi(Type)
bool GetErrorForUi(Type A) {
^~~~~~~~~~~~~
enableif.cpp:26:6: note:   template argument deduction/substitution failed:
enableif.cpp:43:54: note:   couldn't deduce template parameter ‘<anonymous>’
std::cout << GetErrorForUi<Type::First>(Type::First) << std::endl;
^
enableif.cpp:30:6: note: candidate: template<Type T, typename std::enable_if<(! IsTradingSymbol<T>()), int>::type <anonymous> > bool GetErrorForUi(Type)
bool GetErrorForUi(Type A) {
^~~~~~~~~~~~~
enableif.cpp:30:6: note:   template argument deduction/substitution failed:
enableif.cpp:43:54: note:   couldn't deduce template parameter ‘<anonymous>’
std::cout << GetErrorForUi<Type::First>(Type::First) << std::endl;
^
enableif.cpp:44:56: error: no matching function for call to ‘GetErrorForUi<Second>(Type)’
std::cout << GetErrorForUi<Type::Second>(Type::Second) << std::endl;
^
enableif.cpp:26:6: note: candidate: template<Type T, typename std::enable_if<IsTradingSymbol<T>(), int>::type <anonymous> > bool GetErrorForUi(Type)
bool GetErrorForUi(Type A) {
^~~~~~~~~~~~~
enableif.cpp:26:6: note:   template argument deduction/substitution failed:
enableif.cpp:44:56: note:   couldn't deduce template parameter ‘<anonymous>’
std::cout << GetErrorForUi<Type::Second>(Type::Second) << std::endl;
^
enableif.cpp:30:6: note: candidate: template<Type T, typename std::enable_if<(! IsTradingSymbol<T>()), int>::type <anonymous> > bool GetErrorForUi(Type)
bool GetErrorForUi(Type A) {
^~~~~~~~~~~~~
enableif.cpp:30:6: note:   template argument deduction/substitution failed:
enableif.cpp:44:56: note:   couldn't deduce template parameter ‘<anonymous>’
std::cout << GetErrorForUi<Type::Second>(Type::Second) << std::endl;

2( 该代码不是使用icc X86-64(英特尔编译器19.0.1(编译的。该编译器是否不支持std::enable_ifsfinae，或者是英特尔编译器本身的错误？