Why can't I have return types covariant with void*?

Question

Why is the following code a covariance error? Isn't T * covariant with void *...?

struct Base { virtual void *foo(); };
struct Derived : Base { int *foo(); };

GCC says:

invalid covariant return type for 'virtual int* Derived::foo()'

Answer 1

[class.virtual]/p7, emphasis mine:

The return type of an overriding function shall be either identical to the return type of the overridden function or covariant with the classes of the functions. If a function D::f overrides a function B::f, the return types of the functions are covariant if they satisfy the following criteria:

• both are pointers to classes, both are lvalue references to classes, or both are rvalue references to classes [footnote omitted]
• [...]

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From pages 294-5 of D&E:

Afer some consideration of the alternatives, we decided to allow overriding of a B* by a D* and of a B& by a D& where B is an accessible base of D. In addition, const can be added or subtracted wherever that is safe. We decided not to relax the rules to allow technically feasible conversions such as a D to an accessible base B, a D to an X for which D has a conversion, int* to void*, double to int, etc. We felt that the benefits from allowing such conversions through overriding would not outweigh the implementation cost and the potential for confusing users.