Wrong type shown on hover on method with `this.type` as param bound

[Florian3k]

Describe the bug

Write the following code:

val z: Tuple = ???
def m(x: Tuple) = x.zip(z)

Hover on zip It shows:

Symbol signature: inline def zip[This >: Tuple <: Tuple, T2 <: Tuple](t2: T2): Zip[This, T2]

Expected behavior

Expected signature: inline def zip[This >: this.type <: Tuple, T2 <: Tuple](t2: T2): Zip[This, T2] Source: https://github.com/lampepfl/dotty/blob/main/library/src/scala/Tuple.scala#L51

Operating system

macOS

Editor/Extension

VS Code

Version of Metals

0.11.9

Scala version

3.2.0

Extra context or search terms

This problem also occurs in other methods with this.type as param bounds, eg: inline def take[This >: Tuple <: Tuple](n: Int): Take[This, n.type]

[jkciesluk]

More minimal example:

object Main {
  class T {
    def meth@@od[This >: this.type <: T, T2 <: T](t2: T2): T = ???
  }
}

[tgodzik]

Thanks for reporting! I think this is the way it's printed in the compiler actually :thinking: We might need to fix it upstream.

[tgodzik]

Wait, actually why would we want This >: this.type <: Tuple ?

[prolativ]

To make things clear it should be noted that:

• singleton types (like this.type) are distinct from non-singleton types - so e.g.

val x: Tuple = (1, 2, 3)
summon[x.type =:= Tuple]

won't compile. And we cannot lose this distinction in the IDE

• Metals performs substitutions of types which are already fixed in a given context and this makes sense: e.g. this.type in

inline def zip[This >: this.type <: Tuple, T2 <: Tuple](t2: T2): Zip[This, T2]

doesn't tell you much without more context. Putting these two facts together I think the signature on hover in this case should be

inline def zip[This >: x.type <: Tuple, T2 <: Tuple](t2: T2): Zip[This, T2]

The question is whether this still should be called Symbol signature as in the actual signature of zip symbol there is Tuple.this in the bounds

[tgodzik]

I think the bounds are actually printed by the compiler, so this is probably something to fix there.