Inner traits get outer pointer only if needed

[lrytz]

class O:
  def h = 1
  trait T
    // def g = h

object Test:
  def foo(o: O, x: AnyRef) = x match { case _: o.T => "same"; case _ => "other" }
  def main(args: Array[String]): Unit =
    val o = new O
    assert(foo(new O, new o.T { }) == "other")

This fails because T doesn't get an outer method, so the case _: o.T check cannot compare the outer values. The compiler also doesn't warn about that.

Un-commenting the method def g adds an outer method to trait T, which changes the match evaluation.

[SethTisue]

previous discussions in this area include #2156 (2017) and scala/bug#4440 (2011)

"In dotty, if a subclass needs an outer accessor, it gets one" — Martin (so presumably this exception was inadvertent, rather than a design decision)

[lrytz]

Changing this (adding the outer to the trait) would break binary compatibility: the outer in an inner trait is an abstract method. If we haveclass C { trait T }, adding the abstract method to T requires re-compiling all subclasses that extend T.

So if an inner trait is re-compiled with a new version of the compiler, it acquires an abstract outer method, which would show up in MiMa.

[SethTisue]

Unless there is some flaw in our reasoning here, seems like we can't fix this until Scala 4 and the best we can do in 3.x is warn at the pattern match site 😕

(“our” = me, Lukas, @sjrd at the Center today)

[lrytz]

Scala 2.13.11 warns at the pattern:

Test.scala:29: warning: The outer reference in this type test cannot be checked at run time.
  def foo(o: O, x: AnyRef) = x match { case _: o.T => "same"; case _ => "other" }
                                             ^
1 warning