tweak a test to compile on latest Scala 2.13 nightlies

[SethTisue]

fixes #2355

[codecov-io]

Codecov Report

Merging #2359 into 1.2.x will increase coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            1.2.x    #2359      +/-   ##
==========================================
+ Coverage   94.82%   95.01%   +0.18%     
==========================================
  Files         349      349              
  Lines        6009     5993      -16     
  Branches      223      225       +2     
==========================================
- Hits         5698     5694       -4     
+ Misses        311      299      -12

Impacted Files	Coverage Δ
core/src/main/scala/cats/arrow/Compose.scala	66.66% <0%> (-3.34%)	⬇️
...eycats-core/src/main/scala/alleycats/std/map.scala	4.76% <0%> (-1.3%)	⬇️

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[LukaJCB]

LGTM, did this fix the error?

[SethTisue]

did this fix the error?

yes

[ceedubs]

Do we have any insight into why this wasn't required before but is now? I'll admit to never understanding what these Isomorphisms are used for in tests. This change should probably go in, but it makes me wary of whether there's a bigger potential issue.

[kailuowang]

Nit: can we name it isoFunction1?

[SethTisue]

fixed

[kailuowang]

@ceedubs I myself am not familiar with the issue either. I left this comment on the original issue with the little bits that I know of.

Update: Just to summarize, this is one of those cases when scalac refuses to use an implicit def available in scope, rather insists that you should create a local implicit val and call that def explicitly.

[kailuowang]

Although we still don't know the root cause of this issue, but we never do, it's probably a scalac bug..
¯_(ツ)_/¯

[ceedubs]

¯\_(ツ)_/¯

[ceedubs]

Thanks @SethTisue!