template<typename T>
class C;
 
template<typename T>
class CBase
{
private:
    int foo;
    friend class C<T>;
};
 
template <typename T>
class C : public CBase<T>
{
    using CBase<T>::foo;
public:
    void access() { foo = 1; } // есть везде, кроме T = int
};
 
template <>
class C<int> : public CBase<int>
{
    using CBase<int>::foo;
public:
    void funct() { foo = 42; } // есть только для T = int
};
 
int main()
{
    C<void> cv;
    cv.funct();
    C<int> ci;
    ci.access();
    return 0;
}

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