Generics
A related problem to the issue of null and pattern matching is dealing with
patterns mentioning generic types. Currently, it is forbidden to use instanceof
with a non-reifiable type. However, we suspect that Java programmers would
expect the following to work:
ArrayList<Integer> al = ...
if (al matches List<Integer> li) {
...
}
Whereas perhaps it is to be expected that the following is suspect
Object o = ...
if (o matches List<Integer> li) {
// How could we perform this test?
}
The type restatement distinction that we introduced in the previous email for
dealing with null provides a way forward.
More formally, given an expression e matches U u where e has type T:
If T is assignment convertible to U then this is a type restatement pattern
match, and is allowed regardless of the type U (even if it is non-reifiable).
If T is cast convertible to U, but not assignment convertible, then we emit a
warning/error as per the cast conversion rules.
Do we have any other design options here?
