Generic method inference does not support inferring different nullability for multiple calls to same generic method

[msridhar]

Test:

import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;
@NullMarked
public class Test {
  static <T extends @Nullable Object> T id(T t) {
    return t;
  }
  static <T extends @Nullable Object, U extends @Nullable Object> T chooseFirst(T t, U u) {
    return t;
  }
  static void test(@Nullable String s, String t) {
    String u = chooseFirst(id(t), id(s));
    // this is safe but we currently get an error
    u.hashCode();
  }
}

The issue is that within ConstraintSolverImpl we track inference state for type variables on a per-Element basis:

NullAway/nullaway/src/main/java/com/uber/nullaway/generics/ConstraintSolverImpl.java

Line 73 in 2e578d6

private final Map<Element, VarState> vars = new HashMap<>();

But as the above test shows, we may need to infer different nullability for different calls to the same generic method. Probably, we need to refactor our code to track separate inference state for each (element,invocation) pair.