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Drop requirement that self types are closed #16648

Merged
merged 3 commits into from
Jan 12, 2023
Merged

Drop requirement that self types are closed #16648

merged 3 commits into from
Jan 12, 2023

Commits on Jan 11, 2023

  1. Drop requirement that self types are closed 

    scala#702 introduced a requirement that self types are closed. This means

> If trait X has self type S and C is a class symbol of S, then S also conforms
to the self type of C.

An example that violates this requirement is
```scala
trait X { self: Y => } // error: missing requirement: self type Y & X of trait X does not conform to self type Z of required trait Y
trait Y { self: Z => }
trait Z
```
But it's no longer clear what the requirement should achieve. If we let the example above
typecheck and try to implement X with something like
```scala
class C extends X, Y
```
we would at that point get an error saying that `C` does not conform to the self type Z of Y.
So it would have to be
```scala
class C extends X, Y, Z
```
and this one looks fine.

The original change in scala#702 was made to avoid a crash in pending/run/t7933.scala.
Unfortunately, we cannot reproduce this anymore since it depends on nsc.interpreter,
which is no longer part of Scala 2.13.

Since we are no longer sure what the restriction should achieve I think it's better to
drop it for now. If people discover problems with code that uses "open" self types, we
can try to fix those problems, and if that does not work, would fallback re-instituting
the restriction. It's not ideal, but I don't see another way.
    @odersky
    odersky committed Jan 11, 2023
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  2. Add test and drop diagnostic output unless debugging

    @odersky
    odersky committed Jan 11, 2023
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  3. Revert to old definition if isSelfSym

    @odersky
    odersky committed Jan 11, 2023
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