Mark match that is unchecked by Scala 2 as unchecked

[travisbrown]

The Dotty exhaustivity checker is both smarter and more demanding than Scala 2's, and this is a case where it warns while Scala 2 just silently ignores the fact that it can't prove exhaustivity (it's the only case like this in cats-core).

It would be possible to make the match exhaustive in a way that both compilers would accept by writing something like this:

  as match {
    case h ==: t   => f(h, Eval.defer(loop(t)))
    case _ => lb
  }

…but that results in an extra ArrayBuffer instantiation in the empty case. Adding @unchecked seems to me like a reasonable fix, since it just flags what the Scala 2 compiler is already doing.

[codecov-io]

Codecov Report

Merging #3190 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3190   +/-   ##
=======================================
  Coverage   92.99%   92.99%           
=======================================
  Files         376      376           
  Lines        7382     7382           
  Branches      195      201    +6     
=======================================
  Hits         6865     6865           
  Misses        517      517

Flag	Coverage Δ
#scala_version_212	93.32% <ø> (+0.01%)	⬆️
#scala_version_213	90.55% <ø> (-0.03%)	⬇️

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/Chain.scala	98.43% <ø> (ø)	⬆️

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[dwijnand]

Adding @unchecked seems to me like a reasonable fix, since it just flags what the Scala 2 compiler is already doing.

True, but it's also big and loses the value Dotty is (... finally! IMO) adding, there.

but that results in an extra ArrayBuffer instantiation in the empty case

It's a touch of feature creep, but maybe uncons and initLast should fast-track on isEmpty? It would drop that cost.