Restrict the instantiation type on a constructor

It is desirable to specify the result type (the "return type") of a constructor.

For example, new TreeSet<Integer>() has type TreeSet<@NonNull Integer>, but other treesets can permit nullable elements.

More generally, we wish to permit the first two constructor calls but forbid the last two.

   new TreeSet<@Nullable Integer>(new IntegerOrNullComparator()); 
   new TreeSet<Integer>(); 

   //:: error: (type.argument.type.incompatible) 
   new TreeSet<@Nullable Integer>(); 
   //:: error: (type.argument.type.incompatible) 
   new TreeSet<@Nullable Integer>(new IntegerComparator()); 

Java provides syntax for restricting the receiver type on an instance method, but no syntax for type parameter annotations on the constructor result. Only a type annotation, not a full type which would include type parameters, can precede a constructor declaration. So, we would need to make up a new annotation to express the constraint.

There are likely to be classes other than TreeSet that need to utilize this new mechanism.

The issue names no files, tests, or entry points. Start by locating the constructor type-checking and type-annotation handling in the Checker Framework, then compare it with receiver-type restrictions for instance methods. Done means defining a mechanism for constraining constructor result type arguments and checking the four TreeSet examples with the stated diagnostics.