Invalid covariant type with CRTP clonable class

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Even if B is indeed derived from Clonable<B>, the problem here is that Clonable<B> construction is not valid, as it defines 

B* clone() const override

which of course is not an override of AbstractClonable::clone(), since the compiler doesn’t see B at this point as a child of AbstractClonable. So I believe the issue lays in the fact that the compiler cannot build the Clonable<B> base of B.

A workaround (but not really the same as what you want) is to define

Clonable* clone() const override

in Clonable. As you mentioned in the comment, you can also define a free function 

template<typename T> 
T* clone(const T* object) 
{ 
    return static_cast<T*>(object->clone()); 
}

Related: Derived curiously recurring templates and covariance

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