Scala 2 to Scala 3: Instantiating a trait no longer recognises new methods

Question

The following code compiles and runs in Scala 2:

trait SomeTrait {
  val myName: String
}

class SomeClass() extends SomeTrait {
  override val myName: String = "Billy Banana"
}

val someClass = new SomeClass()
println(someClass.myName)

val extendedTrait = new SomeTrait {
  override val myName: String = "Amy Apples"
  def printName(): Unit = println(myName)
}

extendedTrait.printName()

However in Scala 3, there is a compilation error at this line:

extendedTrait.printName()

What in the Scala 3 docs references this change?

Answer 1

Scala 2 used to infer structural types by default, Scala 3 does not.

val foo = new {
  def bar: Unit = ???
}
// Scala 2:
// foo: AnyRef { def bar: Unit }
// Scala 3:
// foo: AnyRef

You'd have to explicitly make it a structural type:

val extendedTrait: SomeTrait {
  def printName(): Unit
} = new SomeTrait {
  override val myName: String = "Amy Apples"
  def printName(): Unit = println(myName)
}