Fix #2099: Make param forwarders override the param they forward to

[smarter]

Previously, in the following code:

class A(val member: Int)
class SubA(member: Int) extends A(member)

We created a forwarder in SubA like this:

  private[this] def member: Int = super.member

Since a private method cannot have the same name as an inherited
non-private one, we subsequently name-mangled the forwarder in
ExpandPrivate. However, this was only done in the TreeTransformer, not
in the DenotTransformer, this lead to separate compilation issues (see
added testcase).

Since we cannot detect that a ParamAccessor is a forwarder in a
DenotTransformer, we cannot fix the problem locally in ExpandPrivate.
Instead, we change ParamForwarding so that the forwarder now looks like:

  override def member: Int = super.member

To make this work we restrict ParamForwarding to only create forwarders
for non-final fields.

[smarter]

Hah! I am smitten by my own diabolical tests. Here's a minimization:

class A(val theValue: Int)
class NonVal(theValue: Int) extends A(theValue) {
  def getTheValueInNonVal = theValue
}
class X(override val theValue: Int) extends NonVal(0)


object Test {
  def main(args: Array[String]): Unit = {
    val x14 = new X(14)

    assert(x14.getTheValueInNonVal == 0)
  }
}

In both scalac and dotty before this PR, the assertion succeeds. But it now fails because calling theValue in NonVal#getTheValueInNonVal is now a virtual call that is overridden in X. @odersky : What do you think of this change?

[odersky]

Yes, I guess 0 is right.

[smarter]

@odersky Then I don't think the proposed approach of making forwarders override the param they forward to can work.

[DarkDimius]

@smarter, I've proposed an approach to name-mangle forwarders while keeping them public.

[smarter]

@DarkDimius This makes things more complicated because we look for an inherited accessor by name in ParamForwarding (see inheritedAccessor), if we name-mangle forwarders we need to look for two names: the name-mangled one and the regular one. It does avoid other problems so this might be the best solution we can have.

[smarter]

@DarkDimius I give up, I've tried various things but I can't get .ensureNotPrivate to work correctly before pickling, too many things break. Could you give it a try?

[DarkDimius]

@smarter, I'll have a look.

[smarter]

Closed in favor of #2106