CODEWITHSUNDEEP
c++c++11templatesvisual-c++
Alex Pai
Alex Pai

Reputation: 125

MSVC:"error C2244: unable to match function definition to an existing declaration" with function refer to type alias of specialized template class

The following code can be compiled with gcc(4.8 and up), while the MSVC (Visual Studio 2017, _MSC_VER 1910) gives error C2244: 'A::func': unable to match function definition to an existing declaration.

#include <iostream>

template<typename T, bool isInt= std::is_integral<T>::value >
class B
{
public:
    using RET = T;
};

template <typename T>
class B <T, false>
{
public:
    using RET = void;
};

template<typename T>
class A
{
    using type = T;
public:
    typename B<type>::RET func();
};

template<typename T>
typename B<typename A<T>::type>::RET A<T>::func()
{
    std::cout << "func" << std::endl;
    return 0;
}

int main()
{
    A<int> a;
    a.func();
    return 0;
}

Full error message from MSVC:

error C2244: 'A<T>::func': unable to match function definition to an existing declaration
note: see declaration of 'A<T>::func'
note: definition
note: 'B<A<T>::type,std::is_integral<T>::value>::RET A<T>::func(void)'
        with
        [
            T=A<T>::type
        ]
note: existing declarations
note: 'B<T,std::is_integral<_Ty>::value>::RET A<T>::func(void)'

It can compile if I eliminate the template specialization in class B. 

template<typename T>
class B
{
public:
    using RET = T;
};

It can also compile if I don't use the type alias in class A.

template<typename T>
class A
{
public:
    typename B<T>::RET func();
};

template<typename T>
typename B<T>::RET A<T>::func()
{
    std::cout << "func" << std::endl;
    return 0;
}

And it can also compile if class A is not template.

class A
{
    using type = int;
public:
    typename B<type>::RET func();
};

It seems that the MSVC does not well support the mix use of template class, type alias and template specialization when separating the class member declaration and definition. I am not sure if there's a special term for this usage. Will the MSVC support this in newer version? Or any existing compile option can fix it?

This issue has been reported to visual studio developer community. https://developercommunity.visualstudio.com/content/problem/225941/error-c2244-unable-to-match-function-definition-to.html

Upvotes: 1

Views: 876

Answers (1)

pem
pem

Reputation: 185

encountered the same problem today. I guess this is a bug in MVSC. In 15.9 it still exists.

Upvotes: 0

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