Lenient mode errors for returning nullable values from null-unmarked methods with a type-parameter return type

[cpovirk]

At the current main-eisop (76b33c9) and still with #171:

class Issue172<E> {
  E e() {
    return null;
  }
}

$ ./demo Issue172.java
Issue172.java:3: error: [return.type.incompatible] incompatible types in return.
    return null;
           ^
  type of expression: null?
  method return type: E
1 error

This should be allowed because the return type should be read as E* (i.e., E with unspecified nullness), which permits null in lenient mode.

Since this error comes up only in null-unmarked code, we should be OK with suppressing the small number of times that it occurs. Still, I'm filing it for tracking.

[cpovirk]

(76b33c9#diff-ce82a59b06d0d82bfc6d06835d1b06ac15619bc3a71617d32189d041f02ca9c0R921 came to mind because I remembered seeing nullnessOperatorUnspecified, but undoing that change does not help.)

[cpovirk]

I've realized that my earlier testing was incomplete. And it turns out that this problem comes up more when I test fully.

On the bright side, my testing with #175 (well, with my variant of it) does appear to eliminate those errors. So we still look to be in good shape.

[cpovirk]

Yes, once #171 and #175 have both landed, I think we'll be ready to switch over.

[cpovirk]

(Or, come to think of it, I might be able to mostly work around this one if necessary. But I'm not sure, and it would be at least a little work.)

[wmdietl]

Fixed by #175.