[mono] Check for additional implemented variant interfaces

[lambdageek]

If a class implements a variant interface, consider whether it is explicitly implementing (as opposed to obtaining by being a subclass of some base class) some variant interfaces.

Two examples:

public interface IFactory<out T> { T Get(); }
public class Foo {}
public class Bar : Foo {}
public class FooFactory : IFactory<Foo> { public Foo Get() => new Foo(); }
public class BarFactory : FooFactory, IFactory<Bar> { public new Bar Get() => new Bar(); }

In this case, BarFactory explicitly implements IFactory<Bar> and also IFactory<Foo>.

Conversely for contravariant gparams:

interface ITaker<in T> { string Consume (T x); }
class Foo {}
class Bar : Foo {}
class BarTaker : ITaker<Bar> { public string Consume (Bar x) => "consumed Bar"; }
class FooTaker : BarTaker, ITaker<Foo> { public string Consume (Foo x) => "consumed Foo"; }

In this case FooTaker implements ITaker<Foo> but alsoITaker<Bar>.

When this happens, the signature of the implementing method 'Bar BarFactory:Get()' doesn't match the signature of the implemented interface method 'Foo IFactory<Foo>:Get()'.

We should check the signature parameters of the candidate method and the implemented method, but I think the interface setup code already checks this for us.

Fixes #48512

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Issue Details

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If a class implements a variant interface, consider whether it is explicitly implementing (as opposed to obtaining by being a subclass of some base class) some variant interfaces.

Two examples:

public interface IFactory<out T> { T Get(); }
public class Foo {}
public class Bar : Foo {}
public class FooFactory : IFactory<Foo> { public Foo Get() => new Foo(); }
public class BarFactory : FooFactory, IFactory<Bar> { public new Bar Get() => new Bar(); }

In this case, BarFactory explicitly implements IFactory<Bar> and also IFactory<Foo>.

Conversely for contravariant gparams:

interface ITaker<in T> { string Consume (T x); }
class Foo {}
class Bar : Foo {}
class BarTaker : ITaker<Bar> { public string Consume (Bar x) => "consumed Bar"; }
class FooTaker : BarTaker, ITaker<Foo> { public string Consume (Foo x) => "consumed Foo"; }

In this case FooTaker implements ITaker<Foo> but alsoITaker<Bar>.

When this happens, the signature of the implementing method 'Bar BarFactory:Get()' doesn't match the signature of the implemented interface method 'Foo IFactory<Foo>:Get()'.

We should check the signature parameters of the candidate method and the implemented method, but I think the interface setup code already checks this for us.

Fixes #48512

Author:	lambdageek
Assignees:	-
Labels:	area-VM-meta-mono
Milestone:	-

[lambdageek]

Test failures are related. I think I'm trashing some unrelated vtable entries. May need a signature check after all.

[lambdageek]

@vargaz @thaystg could you review? In particular, I'm not sure if this is too risky of a hack - I'm not sure if this is close to what the ECMA spec says to do for variant generics.