Prefix not checked in realizability check for non-final lazy val

[abeln]

  1 class Hello {
  2   class Foo {
  3     class Bar
  4     final lazy val s: Bar = ???
  5   }
  6 
  7   lazy val foo: Foo = ???
  8 
  9   val x: foo.s.type = ???
 10   val x2: foo.s.type = ???
 11 }

Expected: both line 9 and 10 produce an error.

Actual: only line 10 produces an error:

10 |  val x2: foo.s.type = ???
   |              ^
   |              Hello.this.foo.Bar(Hello.this.foo.s) is not a legal path
   |              since it refers to nonfinal lazy value foo

The reason is the cached and uncached cases are handled differently in https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/core/CheckRealizable.scala#L80
The uncached case doesn't check the prefix (hence why only the second time we try to use the same invalid path errors out).