Consider the contravariant interface definition with a delegate:

public interface IInterface<in TInput>
{
    delegate int Foo(int x);
    
    void Bar(TInput input);
    
    void Baz(TInput input, Foo foo);
}

The definition of Baz fails with an error:

CS1961
Invalid variance: The type parameter 'TInput' must be covariantly valid on 'IInterface<TInput>.Baz(TInput, IInterface<TInput>.Foo)'. 'TInput' is contravariant.

My question is why? On first glance this should be valid, as the Foo delegate has nothing to do with TInput. I don't know if it's the compiler being overly conservative or if I'm missing something.

Note that normally you wouldn't declare a delegate inside an interface, in particular this doesn't compile on versions older than C# 8, since a delegate in an interface needs default interface implementations.

Is there a way to break the type system if this definition was allowed, or is the compiler conservative?