leaking of local parameter of dependent method type

[scabug]

object Test {
  trait A { a =>
    type X
    val x: a.X
  }
  val a = new A {
    type X = Int
    val x = 1
  }
  def f(arg: A): arg.X = arg.x
  val x = f(a: A)
}

snippet from -Xprint:typer:

...
    private[this] val x: arg.X = Test.this.f((Test.this.a: Test.A));
    <stable> <accessor> def x: arg.X = Test.this.x
...

arg is not in scope...

I don't know yet how to turn this into a true soundness issue, but it's weird.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-7084?orig=1
Reporter: @namin
See #3873

[scabug]

@retronym said:
Reflecting my way to the same conclusions as you reached:

scala> :power
Already in power mode.

scala> val u = rootMirror.universe; import u._, language._, intp.deconstruct.showu: u.type = scala.tools.nsc.interpreter.IMain$$anon$1@4a2e2b5c
import u._
import language._
import intp.deconstruct.show

scala> object Foo { trait A { type X }; def id(a: A): a.X = ???; def foo = id(??? : A) } 
defined module Foo

scala> val fooTpe = typeOf[Foo.type].decl(newTermName("foo")).tpe.resultType
fooTpe: u.Type = a.X

scala> val aSym = fooTpe.prefix.asInstanceOf[SingleType].sym
aSym: u.Symbol = value a

scala> aSym.isValueParameter
res62: Boolean = true

scala> aSym.owner
res63: u.Symbol = method id

I was also unable to poke a soundness hole:

object Foo {
  trait A { trait X { type A; def f(a: A); val a: A }; def X: X };
  object i extends A { object X extends super.X { type A = Int; def f(a: Int) = (); val a = 0 } };
  object s extends A { object X extends super.X { type A = String; def f(a: String) = (); val a = "" } };

  def id(a: A): a.X = a.X
  val x = id(i : A) // x: a.X

  id(i : A).f(id(s : A).a)
  // <console>:47: error: type mismatch;
  // found   : a.X#A
  // required: _31.A where val _31: a.X
  //            id(i : A).f(id(s : A).a)
}

This commit to InstantiateDependentMap is work a look:

scala/scala@b40f4ba322