mutable vs. immutable Seq handling in 2.13 upgrade

[philvarner]

Is your feature request related to a problem? Please describe.

In Scala <= 2.12, a non-fqn Seq referenced the mutable structure scala.collection.Seq[+A]. As of 2.13, this changed so it now references the immutable scala.collection.immutable.Seq[A]. To compile against 2.13, some change needs to be made against the code. What should that change be?

Describe the solution you'd like

I see a few options for this:

1. explicitly change unqualified uses of Seq as a type to be mutable.Seq to preserve mutable semantics. The primary drawback to this is that callers of the methods that have this return type that use unqualified Seq as a return type will also have to update their code to the same when migrating to 2.13.

2. leave the unqualified uses of Seq as a type they are, and add explicit .toSeq on return values. This is a no-op for mutable structures (like ListBuffer) in 2.12, but will have the effect of creating a new container object of type Seq in 2.13.

3. Change parameters to be Iterable (sometimes bad, since a caller could pass an infinite iterable). Leave return types as Seq and add .toSeq on return values as in (2)

Describe alternatives you've considered

n/a

Additional context

n/a

[pomadchin]

I think eveyrhting was resolved in terms of #3294, the rest of warning would probably make sense to remove after dropping Scala 2.12.