Pattern matching with type annotations allows impossible cases

This should not compile:

  sealed trait Foo[T]
  case class Bar[T](s: String)
  
  def shouldError[T](foo: Foo[T]): String = 
    foo match {
      case bar: Bar[T] => bar.s
    }

Not even a warning is produced, as is.

[smarter]

A compromise would be to disallow (foo: Foo) match { case bar: Bar => ... but allow (foo: Foo) match { case bar: Foo & Bar => ...

[smarter]

More complicated example:

sealed trait Foo
trait Child extends Foo
case class Bar(s: String)
class Bla extends Bar("") with Child // This could be in a different file

The only thing we know for sure is that Foo has a single direct child trait Child, but we know nothing about the children of the child trait.