Unresolved symbols assertion during pickling when inlining def with result type with non-object prefix

[milessabin]

Compiling the following,

trait Foo {
  type Id[t] = t
  inline def foo[T](t: T) <: Id[T] =
    inline t match {
      case i: Int => (i+1).asInstanceOf[Id[T]]
      case _ => t
    }
}

object Bar extends Foo

object Test {
  Bar.foo(23)
}

Results in a compiler crash due to an assertion,

exception occurred while compiling tests/pos/inline-from-super.scala
java.lang.AssertionError: assertion failed: unresolved symbols: value Foo_this when pickling tests/pos/inline-from-super.scala while compiling tests/pos/inline-from-super.scala
Exception in thread "main" java.lang.AssertionError: assertion failed: unresolved symbols: value Foo_this when pickling tests/pos/inline-from-super.scala

If we move the definition of Id out of the definition of Foo it compiles,

object Ext {
  type Id[t] = t
}
import Ext._

trait Foo {
  inline def foo[T](t: T) <: Id[T] =
    inline t match {
      case i: Int => (i+1).asInstanceOf[Id[T]]
      case _ => t
    }
}

object Bar extends Foo

object Test {
  Bar.foo(23)
}

It also compiles if we move the entire body of Foo into Bar,

object Bar {
  type Id[t] = t
  inline def foo[T](t: T) <: Id[T] =
    inline t match {
      case i: Int => (i+1).asInstanceOf[Id[T]]
      case _ => t
    }
}

object Test {
  Bar.foo(23)
}

[milessabin]

I'm happy to take a swing at this. I'd be grateful for any pointers.

[odersky]

You get a more detailed message by turning -Ycheck:all (or just -Ycheck:front) on. The issue is that
there is a referenced symbol Foo_this that is not defined in the tree after typer. My guess is that this is due to Inliner.dropUnusedDefs not detecting a reference to Foo_this which would cause it to garbage collect the tree node. In the case it might be an owner of Id that is the undetected reference.

[milessabin]

Thanks.

[milessabin]

PR here: #5646.