Breaking changes between Scalac and Dotty -language:Scala2

[allanrenucci]

This is a meta issue aimed at listing all the breaking changes there are between Scalac and Dotty under Scala2 mode. This should make migration easier.

• Structural types need an explicit import in Dotty

val o: { def foo: Int } = new Object { def foo = 1 } 
o.foo // Error in Dotty

import scala.reflect.Selectable.reflectiveSelectable
o.foo // OK

• Scalac infers structural types

val o = new Object { def foo = 1 } 
o.foo // OK in Scalac

import scala.reflect.Selectable.reflectiveSelectable
o.foo // Error in Dotty

• Overridden members without explicit type inherit type from parent in Dotty

class A { def opt: Option[Int] = None }
class B extends A { override def opt = Some(1) }
val some: Some[Int] = (new B).opt // Error in Dotty

• See Eta-expansion of structural types under Scala2 mode #4026
• Macros are not supported in Dotty
• Introduced new keyword: unused, enum, opaque, augment

[Blaisorblade]

Lots of things under http://dotty.epfl.ch/docs/reference/dropped/ also should count — existential types, type projections in part (though there might still be changes), and so on. Though at least they're already listed. I'm uncomfortable with the amount of code which might be impacted, and with the fact that we don't have an estimate.

[Blaisorblade]

New changes under discussion: #4490 (comment) (lest I forget to add it here later).

[Blaisorblade]

#1473 was also meant to collect such issues. Probably both lists should be merged into a documentation page.

[OlivierBlanvillain]

Merge into https://github.com/lampepfl/dotty/wiki/Known-incompatibilities-with-scalac