`@Repeated` can leak as method result type

[abgruszecki]

We probably should drop the @Repeated annotation when inferring the result type of a method:

scala> def foo(s: String*) = s
def foo(s: String*): Seq[String] @Repeated

scala> def foo(s: String*) = { val s1 = s; s1 }
def foo(s: String*): Seq[String]

[abgruszecki]

Further problems:

scala> def foo(s: String*) = Option(s)
def foo(s: String*): Option[Seq[String] @Repeated]

scala> def foo(s: String*) = {val o = Option(s); o }
def foo(s: String*): Option[Seq[String] @Repeated]

Note that in the second case we also infer a type containing @Repeated for a local variable.

[abgruszecki]

This may be fixed in avoid at basically no cost - since we already traverse the type, we might as well strip the annotation while we're at it (we do use ApproximatingTypeMap there, but I don't think it matters). This does mean that local variables can have @Repeated inferred, but I'm uncertain if that matters.

A cleaner fix might be simply to make the body of the method see vararg arguments w/o the annotation, but I'm unsure if that can be easily accomplished.

[smarter]

My hunch would be that https://github.com/lampepfl/dotty/blob/54fb992e81e580b47614674c3d2eea4a6a4a7a85/compiler/src/dotty/tools/dotc/core/ConstraintHandling.scala#L281 should handle this.