+= is not a member of Set[String]

[DarkDimius]

[info] # partest /Users/dark/workspace/dotty/tests/partest-generated/run/colltest1.scala
[info] % dotc colltest1.scala
[info]
[info] compiling /Users/dark/workspace/dotty/tests/partest-generated/run/colltest1.scala
[info] options: -pagewidth 160 -Yno-deep-subtypes -Yno-double-bindings -Ycheck:tailrec,resolveSuper,mixin,restoreScopes -d ./tests/partest-generated/run/colltest1-run.obj
[info] warning: encountered F-bounded higher-kinded type parameters for type scala$collection$generic$GenSetFactory$$CC; assuming they are invariant
[info] warning: encountered F-bounded higher-kinded type parameters for type scala$collection$generic$SetFactory$$CC; assuming they are invariant
[info] /Users/dark/workspace/dotty/tests/partest-generated/run/colltest1.scala:148: error: += is not a member of scala.collection.Set[String]
[info]     s += ("D", "E", "F")