wrong "fruitless type test" warning

[scf37]

scalac seems to think that function argument type can not implement local trait but that is not true.

See an example:
https://scastie.scala-lang.org/UvXkI2YiRJi8uXePPp1cHg

Reproduced on 2.12.8, 2.13.0

[martijnhoekstra]

Shorter:

trait A
def f(v: A): Unit = {
  trait Y
  if (v.isInstanceOf[Y]) println("Yes it can be an Y")
  else f(new A with Y)
}

f(new A{})

[Jasper-M]

trait A
def f(v: A): Unit = {
  trait Y
  val ay = new A with Y
  if (v.isInstanceOf[Y]) println("Yes it can be an Y") //no warning now
  else f(ay)
}

trait Y is determined as effectively final because symbolOf[Y].children is empty until after new A with Y.

https://github.com/scala/scala/blob/v2.13.0/src/compiler/scala/tools/nsc/typechecker/Checkable.scala#L250
https://github.com/scala/scala/blob/v2.13.0/src/reflect/scala/reflect/internal/Symbols.scala#L1024

[SethTisue]

Scala 3.3.1-RC5:

4 |  if (v.isInstanceOf[Y]) println("Yes it can be an Y")
  |      ^
  |the type test for Y cannot be checked at runtime because it's a local class

I'm not completely sure I understand the warning message here — I guess it's because each invocation of f gives a different Y, so we can't know at runtime which Y we've got