In fact, leave Nosica invariant, but allow implementation of multi methods ...
That is,

interface B {}
interface A {
sub f(B b);
}
class AImpl implements A {
public sub f(BImpl b) {} // compil time error, covariance is not allowed
}
class AImpl2 implements A {
public sub f(B b) {} // OK interface is implemented (invariance)
public sub f(BImpl b) {} // this is a specialisation of the implementation of
method A.f(B) : a multi method
}

B b = new BImpl();
A a = new AImpl2();
a.f(b); // will call AImpl2.f(BImpl) through multiple dispatch

As the language is invariant, method f(B) must be implemented ... This is a type
safe compiler construct. (cannot fail at runtime).
As the language allows multi methods (via multiple dispatch), we allow a kind of
covariance that is much powerfull ...

That is we have the best of the two worlds ...
See forum for details on implementation ...