Make CompatibilityBEI2.java pass.

[xingweitian]

This PR makes CompatibilityBEI2.java pass.

Here is my thinking:

In @Immutable interface ImmutableInterface, after the viewpoint adaption, the type of Object is @Immutable.
So, <@Mutable Object> is incorrect (@Mutable is not a subtype of @Immutable).

Similarly, <@ReceiverDependantMutable Object> is incorrect (@RDM is not a subtype of @Immutable).

[wmdietl]

For the interface

@Immutable interface ImmutableInterface<E extends @ReceiverDependantMutable Object> {}

are the only valid type arguments @Immutable?

Can we easily add an error on the declaration of E's bound that the @ReceiverDependantMutable is misleading and that @Immutable must be used instead?

[wmdietl]

Your argument makes sense to me.

[xingweitian]

For the interface

@Immutable interface ImmutableInterface<E extends @ReceiverDependantMutable Object> {}

are the only valid type arguments @Immutable?

Can we easily add an error on the declaration of E's bound that the @ReceiverDependantMutable is misleading and that @Immutable must be used instead?

For

@Immutable interface ImmutableInterface<E extends @ReceiverDependantMutable Object> {}

the valid type can also be @Mutable, that is to say, if we have

@Immutable interface ImmutableInterface<E extends @Mutable Object> {}

after the viewpoint adaption, the type of Object will not be changed.

[wmdietl]

For

@Immutable interface ImmutableInterface<E extends @ReceiverDependantMutable Object> {}

the valid type can also be @Mutable, that is to say, if we have

@Immutable interface ImmutableInterface<E extends @Mutable Object> {}

after the viewpoint adaption, the type of Object will not be changed.

You are changing the bound in the example.
Take the fixed interface:

@Immutable interface ImmutableInterface<E extends @ReceiverDependantMutable Object> {}

Which of the following are well-formed types:

ImmutableInterface<@Mutable String> a;
ImmutableInterface<@Immutable String> b;
ImmutableInterface<@Readonly String> c;
ImmutableInterface<@ReceiverDependableMutable String> d;

?

[xingweitian]

For

@Immutable interface ImmutableInterface<E extends @ReceiverDependantMutable Object> {}

the valid type can also be @Mutable, that is to say, if we have

@Immutable interface ImmutableInterface<E extends @Mutable Object> {}

after the viewpoint adaption, the type of Object will not be changed.

You are changing the bound in the example.
Take the fixed interface:

@Immutable interface ImmutableInterface<E extends @ReceiverDependantMutable Object> {}

Which of the following are well-formed types:

ImmutableInterface<@Mutable String> a;
ImmutableInterface<@Immutable String> b;
ImmutableInterface<@Readonly String> c;
ImmutableInterface<@ReceiverDependableMutable String> d;

?

Sorry, I misunderstand before. For this fixed interface, only @Immutable is correct.

[wmdietl]

So when the checker sees

@Immutable interface ImmutableInterface<E extends @ReceiverDependantMutable Object> {}
@Mutable interface MutableInterface<E extends @ReceiverDependantMutable Object> {}

it would be convenient to suggest:

@Immutable interface ImmutableInterface<E extends @Immutable Object> {}
@Mutable interface MutableInterface<E extends @Mutable Object> {}

right? Maybe file this as a separate issue that you can look at sometime in the future.

[xingweitian]

#9 was created for the further improvement , I will merge this PR for now.