Fix #1990: Handle inlining where this proxies change types

[odersky]

The new situation in the test was that outer of the inlined method
was A but its as-seen-from type is a subtype B. Outer path logic
needs to be adapted to that case.

[smarter]

Looks like this parameter is always set to true, is that intended?

[odersky]

No, that was an oversight.

[smarter]

But we always have outOfContext = true in transformSelect, not just in code that has been inlined.
Shouldn't the condition be something like !tpd.enclosingInlineds.isEmpty instead?

[odersky]

Note that C_<OUTER> nodes are only created for inline proxies. The code in question only applies to these.

[smarter]

Oh, I missed that, makes sense now.

[smarter]

I found the fix a bit suspicious so I played a bit with the testcase and got it to crash again:

class A { self =>
  class Foo {
    inline def inlineMeth: Unit = {
      println(self)
    }
  }
}

class C extends A {
  class B extends A
}

object Test {
  def main(args: Array[String]): Unit = {
    val c = new C
    val b = new c.B

    (new b.Foo).inlineMeth
  }
}

Stack trace: https://gist.github.com/smarter/75ee9b3da1ffe127721bb93535bd8483

[smarter]

Progress! Now I can only get it to crash at runtime, I apologize for the devious code :).

trait A { self =>
  class Foo {
    inline def inlineMeth: Unit = {
      println(self)
    }
  }
}

class A2 extends A {
  class C extends Foo with A

  val c = new C
  (new c.Foo).inlineMeth
}

object Test {
  def main(args: Array[String]): Unit = {
    new A2
  }
}

$ dotr Test
Exception in thread "main" java.lang.ClassCastException: A$Foo cannot be cast to A2$C
        at A2.<init>(i1990a.scala:13)
        at Test$.main(i1990a.scala:18)
        at Test.main(i1990a.scala)

The crash happens because the outer path stops too early:

        val Foo_this: A2.this.c.Foo = new A2.this.c.Foo()
        val A_this: A2.this.C(A2.this.c) = 
          Foo_this.asInstanceOf[A2.this.C(A2.this.c)]

If I replace class C extends Foo with A by class C extends A, the path is correct:

        val Foo_this: A2.this.c.Foo = new A2.this.c.Foo()
        val A_this: A2.this.C(A2.this.c) = 
          Foo_this.A$Foo$$$outer.asInstanceOf[A2.this.C(A2.this.c)]

[DarkDimius]

Those bugs are very similar from ones that we had in linker.
here's another test for you to try:

trait A { self =>
  type T
  val field: T
  def bar(f: T): T
  class Foo {
    inline def inlineMeth: T = {
      bar(field)
    }
  }
}

class A2 extends A {
  type T = Int
  val field = 1
  override def bar(a: Int) = a + 1
  class C extends Foo with A

  val c = new C
  val ret = bar((new c.Foo).inlineMeth) // how many boxings? 
  val resI: Int = ret
}

object Test {
  def main(args: Array[String]): Unit = {
    new A2
  }
}

[odersky]

@DarkDimius That test does not typecheck. Is that intentional?

[smarter]

LGTM!