Does JLS tell if this program is legal?

[Stephan Herrmann]

Thanks, Dan,

Am 22.05.24 um 02:13 schrieb Dan Smith:
>> On May 21, 2024, at 12:49 PM, Stephan Herrmann <stephan.herrmann at berlin.de> wrote:
>>
>> * Super<Inner<?>> is a supertype of T / One<?> (directly)
> 
> Note that "is a supertype" here isn't true though:
> 
> Is One<?> <: Super<Inner<?>>
> Is One<CAP> <: Super<Inner<?>>
> Is Super<Inner<CAP>> <: Super<Inner<?>>
> Is Inner<CAP> = Inner<?>
> false

Wow, another corner I hadn't yet looked at. :) I had assumed that capture 
applies only when something is being accessed / evaluated. Going from a type to 
its supertype didn't look like evaluation to me.

But now looking at §4.10.2 I see it speaking of capture indeed.

So that would change my analysis
from Super<Inner<?>> is a supertype of T / One<?>
to   Super<Inner<CAP>> is a supertype of T / One<?>

and we continue:
* S1 = Inner<X>
* T1 = Inner<CAP>
* new constraint formula created: ⟨Inner<X> = Inner<CAP>⟩

This, too, reduces to false, and again I would be without an explanation how 
'accept' could be correct.

> This is related to
> https://bugs.openjdk.org/browse/JDK-8016196
> 
> Unfortunately, JLS doesn't specify a way to map from subtypes to supertypes when wildcards are involved. (It's one of those unexplained corners of the type system.)

Isn't that given by that sentence I discovered just now?
	"Given a generic class or interface C with type parameters F1,...,Fn (n > 0), 
the direct supertypes of the parameterized type C<R1,...,Rn> where at least one 
of the Ri (1 ≤ i ≤ n) is a wildcard type argument, are the direct supertypes of 
the parameterized type C<X1,...,Xn> which is the result of applying capture 
conversion to C<R1,...,Rn> (§5.1.10)."

> I've got a solution in my mind that involves the upward projection operation we introduced in 4.10.5: capture, find the supertype, then project away the capture variable. But there are details to work out.

If I apply this strategy in ecj I get these pairs of supertypes of One<?> , Y
* One<?>, One<X>
* Super<? extends Inner<?>>, Super<Inner<X>>

In both pairs we find a wildcard as a direct type argument and hence NO new 
constraint would be generated in §18.3.1.

Is that what you have in mind?

Does javac already implement this application of projection?

thanks,
Stephan