Backport "Be still more careful when computing denotations of class parameters" #16196
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Be still more careful when intersecting info and arguments of a class type parameter.
This is the latest installment of a never-ending story.
The problem is this: Given
```scala
class C[T >: L1 <: H1] { val x: T }
def f(): C[? >: L2 <: H2]
```
what is the type of `f().x`?
With capture conversion (an extremely tricky aspect of the type system forced on us
since Java does it), the type is something like `?1.T` where `?1` is a skolem variable
of type `C[? >: L2 <: H2]`. OK, but what is the underlying (widened) type of `?1.T`?
We used to say it's `C[T >: L1 <: H1]`. I.e. we forgot about the actual arguments.
But then we had to change untupling desugarings from defs to vals in scala#14816 to fix
scala#14783 and it turned out that was not good enough, we needed the information of the
actual arguments, i.e. the type should be `C[T >: L2 <: H2]. Then something else started
failing which relied on the formal arguiment bounds being preserved. So the new resolution
was that the type would be the intersection of the formal parameter bounds and the actual
bounds, i.e. `C[T >: L1 | L2 <: H1 & H2]`. Then there was a series of problems where _that_
failed, either because the parameter bound was F-bounded or because the intersection was an
empty type. The latest installment is that the parameter bounds refer to another parameter
in the same class, which requires a simultaneous substitution of all skolemized bounds for
all dependent parameter references in the parameter bounds. But that's impossible or at least
very hard to achieve since we are looking at the type of a single parameter after capture conversion here.
So the current solution is to also back out of the intersection if there are cross-parameter
references and to use just the argument bounds instead.
12 tasks
szymon-rd
approved these changes
Oct 17, 2022
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Backports #16112