support models of generic types

[uber]

support models of generic types

[msridhar]

Couple notes based on some initial tinkering:

• We'll need to handle assignments, e.g., you can't assign a Pair<@Nullable String, String> to Pair<String, String> (with both @NonNull). Probably just want to support invariance.
• Without inference, ergonomics will be a bit rough. E.g., if you write Pair<@Nullable String, String> p = new Pair<>(null, "hello"); I don't think javac's inference will tell us that the first type parameter for the new expression is @Nullable. Would be a bit unfortunate to have to write Pair<@Nullable String, String> p = new Pair<@Nullable String, String>(null, "hello"); but supporting inference could be a lot more work.

Overall I think adding basic support (even without inference) will be a moderate amount of work to get right.

[msridhar]

FWIW Error Prone has some support for generics and null checking now, implemented by @bennostein. See the inference code and this commit. Perhaps we could do something similar in NullAway? @lazaroclapp