Type variables and wildcards nullability

[beikov]

I tried to simplify a problem that I have in my code base with the type checking of Checker Framework. Here is the Java code:

	interface Base<A, B extends A> {}
	static class Sub<A, B extends A> implements Base<A, B> {}
	static class Main {
		List<Base<?,?>> list;
	}

and this is the error I get when compiling.

error: [type.argument] incompatible type argument for type parameter B of Base.
                List<Base<?,?>> list;
                            ^
  found   : @Initialized @NonNull Object
  required: capture#01 extends @Initialized @Nullable Object

How am I supposed to annotate this to "just work" correctly?

Might be related to #7210

[beikov]

If anyone else runs into this problem, I was able to fix (workaround?) this issue by annotating the wildcard with @Nullable:

	interface Base<A, B extends A> {}
	static class Sub<A, B extends A> implements Base<A, B> {}
	static class Main {
		List<Base<?, @Nullable ?>> list;
	}

I can't quite explain why this works, but it does. Hopefully someone from the Checker Framework maintainers can shed some light on this.

[smillst]

This is a bug. (The original code should type check.) I'm working on a fix.