Type inference: bug or feature?

[Justin Dekeyser]

Dear all,

I'm not sure but I think I've found a bug in Java type inference mechanism.
It may also be a feature, so I'll expose the problem to you below; in terms
of Integer, Number and List, although you'll quickly realize it will work
wrong in any similar situation.

Let's assume I have

static <U, V extends U> List<U> emptyList(Class<U> magnet) {
   return Collections.emptyList();
}

Then the following codes does not compile (for the same reason):

var x = (List<Integer>) emptyList(Number.class);
List<Integer> x = (List<Integer>) emptyList(Number.class);

incompatible types: List<Number> cannot be converted to List<Integer>

however, the following do compile:

var x = emptyList(Number.class); // inferred to List<Number>
List<Integer> x = emptyList(Number.class); // no mistake here, it's Integer
on the left

Is this the expected behavior? Why does casting operation here interfere
with type inference like this?

Regards,

Justin Dekeyser




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