模板类方法实例化当基类中的虚拟无关方法导致MSVC上的编译失败时

Question

Is the following code legal C++?

MS Visual C++ fails, but gcc and clang are fine: https://godbolt.org/z/vsQOaW

It could be an msvc bug, but wanted to check first:

struct Base {
    virtual void junk() = 0;
};

template <class T>
struct Derived : Base {

    void junk() override {
        T::junkImpl();
    }

    void otherMethod() {
    }
};


template <class T>
struct NotDerived {

    void junk() {
        T::junkImpl();
    }

    void otherMethod() {
    }
};


struct TypeWithJunk {
    void junkImpl() {
    }
};

struct TypeWithoutJunk {};


void reproduce(NotDerived<TypeWithoutJunk>* ndt, Derived<TypeWithoutJunk>* dt) {

    // works - junk is not used, not instantiated
    ndt->otherMethod();

    // fails on MSVC - junk is instantiated even if not used
    dt->otherMethod();
}

Answer 1

junk may get instantiated just like the rest of virtual functions because it is required to populate vtable. So all the compilers seem to demonstrate conforming behavior:

17.8.1 Implicit instantiation [temp.inst]

9 … It is unspecified whether or not an implementation implicitly instantiates a virtual member function of a class template if the virtual member function would not otherwise be instantiated.