Don't generate paths with outer accessors to stable references

[odersky]

In that case we can generate directly a reference without going though
an inner this proxy and an outer accessor.

This optimization is necessary since outer accessors are not generated for objects.

Fixes #11914.

[odersky]

I think this would be good to get into RC2. I don't see a particular risk, and it avoids an annoying crash in code generation.

[liufengyun]

LGTM

[liufengyun]

Since we are at the code, I've a question regarding the code just above L511-L517:

    val sortedProxies = thisProxy.toList.map {
      case (cls, proxy) =>
        // The class that the this-proxy `selfSym` represents
        def classOf(selfSym: Symbol) = selfSym.info.classSymbol
        // The total nesting depth of the class represented by `selfSym`.
        def outerLevel(selfSym: Symbol): Int = classOf(selfSym).ownersIterator.length
        (outerLevel(cls), proxy.symbol)
    }.sortBy(-_._1)

It seems to me that the call classOf(selfSym) is useless, as selfSym is already a class symbol.

[liufengyun]

The following line (L531) for static objects look dubious, as it is not effective if lastSelf.exists:

        else if rhsClsSym.is(Module) && rhsClsSym.isStatic then

I expect it to be before the branch lastSelf.exists, thus always generate optimized code.
Currently, it's not the case, as can be seen from the example below:

object Wrapper:
  type E <: Throwable
  class C:
    object syntax:
        extension [A <: Matchable](a: E | A)
          transparent inline def foreach(f: A => Any): Unit =
            a match
              case e: E =>
                Wrapper.this.n + m
              case a: A => f(a)
        val m: Int = 4

  def _catch[A](a: => A): E | A =
    try a
    catch case e: E => e

  val n: Int = 3

object throwables extends Wrapper.C
import throwables.syntax.*

@main def Test(): Unit =
  for x <- Wrapper._catch(1/1) do println(x)

The proxy generated for Wrapper is the following:

        val Wrapper$_this: (Wrapper : Wrapper.type) = syntax$_this.2_<outer>

And the example crashes the compiler with the current PR.

[odersky]

Thanks for these insights!