Fix #4247: Do not crash if self is of Array type

[nicolasstucki]

No description provided.

[liufengyun]

Scalac allows both the following:

class Foo[U] { self : Array[U] => self(0) }

class Foo[U] { self : Array[U] with Nothing =>
  self.length
}

We need to justify why Dotty should deviate in such cases.

[nicolasstucki]

We will never be able to instantiate a Foo because Array is final. The code is nonsensical as no subclass of Foo can mixin Array.

[liufengyun]

I'm thinking it's the same case as def f(x: Nothing): Unit = 3. The code is valid, though the function can not be called. If we have checks for realization of self types during instantiation, can we just reuse that mechanism?

[nicolasstucki]

Maybe we could. But this patch should still be there to ensure there are no compiler crashes on arrays in case that checker fails to detect it early.

Also, It is possible that we may create some other kind of unrealisable array with something that is not the self type.

[allanrenucci]

I tend to agree with @liufengyun. We should fix https://github.com/lampepfl/dotty/blob/4c8a2bd319437f7ab22303a162f5922bc000134d/compiler/src/dotty/tools/dotc/typer/Checking.scala#L148
Otherwise we will reject class Foo[U] { self : Array[U] & Nothing => self(0) } but accept class Foo[U] { self : Array[U] & Nothing => }.

Also, It is possible that we may create some other kind of unrealisable array with something that is not the self type.

Wouldn't that be a bug? Then crashing is the right think to do

[nicolasstucki]

That was a different issue.

[nicolasstucki]

The & Nothing is unrelated tho this particular issue.

[allanrenucci]

That was a different issue

Sorry, my point was that we should also reject test[U](x: Array[U] & Nothing) = x(0), if we reject self: Array[U] & Nothing => self(0)

[Blaisorblade]

If we have checks for realization of self types during instantiation, can we just reuse that mechanism?

BTW, those checks on instantiation are spec-mandated (#4252), so I think I agree with @allanrenucci and @liufengyun.

[nicolasstucki]

The check on instantiation are done and work as expected. The issue is that for this particular case we do have a problem before instantiation. This is a particular exception due to the way we erase Arrays into java arrays.

Additionally the self type Array is completely valid as far as this class is conserened and could be realizable. Though Array would be the only class that could extend it.

[nicolasstucki]

Rebased

[odersky]

Not sure about this one. It seems that we are masking a failure condition by an error, which can get us intro trouble easily. I'd prefer if we found and avoided the root cause instead.