Use ArraySeq.untagged instead of ClassTag[Nothing]

[travisbrown]

Another fairly trivial change that fixes Dotty breakage and avoids some arguably weird behavior on Scala 2. The code this touches has never been released and in any case these changes shouldn't affect the behavior of these implementations.

On Scala 2 you can write something like this:

scala> import scala.collection.immutable.ArraySeq
import scala.collection.immutable.ArraySeq

scala> def empty[A]: ArraySeq[A] = ArraySeq.empty
empty: [A]=> scala.collection.immutable.ArraySeq[A]

But it doesn't work on Dotty:

scala> import scala.collection.immutable.ArraySeq                                                                                                                                                                                            

scala> def empty[A]: ArraySeq[A] = ArraySeq.empty
1 |def empty[A]: ArraySeq[A] = ArraySeq.empty
  |                                          ^
  |                                          No ClassTag available for A

Which is similar to what happens on Scala 2 if you don't let the type parameter get inferred:

scala> def empty[A]: ArraySeq[A] = ArraySeq.empty[A]
                                                 ^
       error: No ClassTag available for A

I think what's happening on Scala 2 is that Nothing is inferred and this implicit ClassTag[Nothing] is plugged in:

scala> implicitly[reflect.ClassTag[Nothing]]
res0: scala.reflect.ClassTag[Nothing] = Nothing

Dotty's scala.reflect doesn't provide this instance:

scala> implicitly[reflect.ClassTag[Nothing]]                                                                                                                                                                                                 
1 |implicitly[reflect.ClassTag[Nothing]]
  |                                     ^
  |                                     No ClassTag available for Nothing

Changes to these two lines fixes the issue on Dotty by using the untagged factory, which uses a ClassTag[Any] behind the scenes.

[codecov-io]

Codecov Report

Merging #3324 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3324   +/-   ##
=======================================
  Coverage   93.31%   93.31%           
=======================================
  Files         378      378           
  Lines        7684     7684           
  Branches      233      233           
=======================================
  Hits         7170     7170           
  Misses        514      514

Flag	Coverage Δ
#scala_version_212	93.38% <ø> (ø)	⬆️
#scala_version_213	93.09% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
...src/main/scala-2.13+/cats/instances/arraySeq.scala	100% <100%> (ø)	⬆️

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