Question

I don’t know why the following scala code can not be compiled:

import collection.immutable.Seq
def foo(nodes: Seq[Int]) = null
val nodes:IndexedSeq[Int] = null
foo(nodes)

=>

error: type mismatch;
 found   : IndexedSeq[Int]
 required: scala.collection.immutable.Seq[Int]
             foo(nodes)
                 ^

In scala-library, IndexedSeq is declared:

trait IndexedSeq[+A] extends Seq[A]...

Answer 1

There are several IndexedSeq traits. Default is scala.collection.IndexedSeq. if you import collection.immutable.IndexedSeq then scala will compile successfully. (Copied from OP)