Scala 2.13からscala.SeqはImmutableになった

Scala 2.12まで

Welcome to Scala 2.12.8 (OpenJDK 64-Bit Server VM, Java 12.0.1).
Type in expressions for evaluation. Or try :help.

scala> val seq1: scala.Seq[Int] = scala.collection.immutable.Seq(1, 2)
seq1: Seq[Int] = List(1, 2)

scala> val seq2: scala.Seq[Int] = scala.collection.mutable.Seq(1, 2)
seq2: Seq[Int] = ArrayBuffer(1, 2)

Scala 2.13から

Welcome to Scala 2.13.0-RC3 (OpenJDK 64-Bit Server VM, Java 12.0.1).
Type in expressions for evaluation. Or try :help.

scala> val seq1: scala.Seq[Int] = scala.collection.immutable.Seq(1, 2)
seq1: Seq[Int] = List(1, 2)

scala> val seq2: scala.Seq[Int] = scala.collection.mutable.Seq(1, 2)
                                                              ^
       error: type mismatch;
        found   : Seq[Int] (in scala.collection.mutable)
        required: Seq[Int] (in scala.collection.immutable)

もうmutable.Seqは代入できない…っていうか今までできたんだ…知らなかった…