This doesn't appear to be a regression, it didn't work with 2.10.0 either.

Ultimately, the problem is inferring the type parameter `C[_]` in the `arbContainer` implicit in Scalacheck. Using the type alias leads the compiler down the wrong path, and it doesn't try all combinations of dealiasing or base classes due to https://issues.scala-lang.org/browse/SI-2712.