"explicit template argument list not allowed" with g++, but compiles with clang++?

user_4625909 0
Viewed 111

I have test code as below.

#include <iostream>
#include <vector>

using namespace std;

template<typename Cont>
class Test
{
    template<typename T, typename = void> static constexpr bool check = false;
    template<typename T>
    static constexpr bool check<T, std::void_t<typename T::iterator>> = true;

public:
    static bool fun()
    {
        return check<Cont>;
    }
};

int main([[maybe_unused]] int argc, [[maybe_unused]] char *argv[])
{
    cout << Test<vector<int>>::fun() << endl;
    cout << Test<int>::fun() << endl;

    return 0;
}

Compile with g++, compiler will complain:

test.cpp:12:27: error: explicit template argument list not allowed
   12 |     static constexpr bool check<T, std::void_t<typename T::iterator>> = true;
      |                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

But the clang++ compiles the code without any error.

What does the error g++ thorws mean? How can I modify the code that both g++ and clang++ compiles it?

Thanks!

templates c++
1 Answers

It should be ok to write a partial specialization in any context

It is listed as a gcc bug but it is not fixed yet: gcc bug

As a workaround you can place the specialization outside class context like:

template<typename Cont>
class Test 
{
    template<typename T, typename = void> static constexpr bool check = false;

public:
    static bool fun()
    {    
        return check<Cont>;
    }    
};
template<typename Cont>
template<typename T>
constexpr bool Test<Cont>::check<T, std::void_t<typename T::iterator>> = true;

int main([[maybe_unused]] int argc, [[maybe_unused]] char *argv[])
{
    cout << Test<vector<int>>::fun() << endl;
    cout << Test<int>::fun() << endl;

    return 0;
}
user_8785320
Related