Making a constructor friend is possible in C++:

class A {
    A() { B b; }
    class B {
        B(){};
        friend A::A();
    };
};

How can I achieve the same but with a template class:

template< typename T >
class EnforceOnlyOneIntConstructor
{
    EnforceOnlyOneIntConstructor() {}
    //friend T::T(int);
    //friend T::OnlyOneIntConstructor(int);
    friend void T::regularFun(int);
};

class OnlyOneIntConstructor
{
    //EnforceOnlyOneIntConstructor< OnlyOneIntConstructor > m_trick;
public:
    OnlyOneIntConstructor(int) //: m_trick{}
    {}

    void regularFun(int) {
        EnforceOnlyOneIntConstructor< OnlyOneIntConstructor > m_trick;
    }

};

int main() {
    OnlyOneIntConstructor a(10);
}

I don’t know if it is even possible. Without commenting out the problematic sections the compiler gives an error:
error: C++ requires a type specifier for all declarations

Source: Windows Questions C++