Add two casts to satisfy Dotty

[travisbrown]

These two casts are necessary to compile cats-core with Dotty. I've been using them locally in my cross-compilation experiments, but wanted to understand for myself why Dotty wants them before opening a PR.

I've just been taking a closer look and have convinced myself that in both cases Dotty is doing the right thing, and Scala 2 only compiles the current code because of soundness bugs.

In AndThen, the issue is that the compiler is inferring the E to be Any in the Concat case, which isn't sound in general. Consider the following simplification:

scala> case class Foo[A](f: A => Any)
defined class Foo

scala> def whatever(x: Any) = x match { case Foo(f) => f("") }
                                                         ^
       error: type mismatch;
        found   : String("")
        required: Nothing

Inferring Nothing here is the safe thing to do, and this failure makes sense. You can trick the compiler into inferring the A to be Any, though, simply by having it appear in a non-contravariant position in another member:

scala> case class Foo[A](a: A, f: A => Any)
defined class Foo

scala> def whatever(x: Any) = x match { case Foo(_, f) => f("") }
whatever: (x: Any)Any

…which will fail at runtime on e.g. Foo[Int]:

scala> whatever(Foo[Int](1, identity))
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
  at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:100)
  at scala.runtime.java8.JFunction1$mcII$sp.apply(JFunction1$mcII$sp.scala:17)
  at .whatever(<console>:1)
  ... 29 elided

Dotty fixes this and refuses to compile the second Foo, and it fails on AndThen for the same reason:

[error] -- [E007] Type Mismatch Error: /home/travis/projects/cats/core/src/main/scala/cats/data/AndThen.scala:125:32 
[error] 125 |          right = inner.andThenF(right)
[error]     |                  ^^^^^^^^^^^^^^^^^^^^^
[error]     |                  Found:    cats.data.AndThen[E$4, Any]
[error]     |                  Required: cats.data.AndThen[Any, Any]

The Eval code also compiles only because the Scala 2 compiler is too willing to infer Any. Here's a simplified example that shows the unsoundness:

trait Foo[+A] {
  def result: Option[A]
}

case class Bar[A]() extends Foo[A] {
  var result: Option[A] = None
}

def whatever(foo: Foo[Any]): Unit = foo match {
  case b @ Bar() => b.result = Some("foo")
}

And then:

scala> val bar = Bar[Int]()
bar: Bar[Int] = Bar()

scala> whatever(bar)

scala> bar.result.get
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
  at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:100)
  ... 29 elided

In context both of these cases are optimizations where we're already doing a lot of casting, and personally I think it's reasonable to add two more casts to satisfy the Dotty compiler.

[codecov-io]

Codecov Report

Merging #3230 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3230   +/-   ##
=======================================
  Coverage   93.05%   93.05%           
=======================================
  Files         376      376           
  Lines        7412     7412           
  Branches      192      200    +8     
=======================================
  Hits         6897     6897           
  Misses        515      515

Flag	Coverage Δ
#scala_version_212	93.38% <100%> (ø)	⬆️
#scala_version_213	92.82% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
core/src/main/scala/cats/Eval.scala	98.83% <100%> (ø)	⬆️
core/src/main/scala/cats/data/AndThen.scala	94.33% <100%> (ø)	⬆️

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[rossabaker]

Nice write-up!