Extension methods shouldn't create extension methods for Scala2x defined symbols

[DarkDimius]

Scala2x has extmethods phase before pickler, so extension methods are picked, and unpicked by dotty.
Ie dotty has already unpickled for RichInt$

    public final Range until$extension0(int $this, int end)
    public final Range until$extension1(int $this, int end, int step)

And it additionally creates(in ExtensionMethods.transform)

denot1: final def until$extension1:
  ($this: scala.runtime.RichInt)(end: Int)scala.collection.immutable.Range
denot1: final def until$extension0:
  ($this: scala.runtime.RichInt)(end: Int, step: Int)scala.collection.immutable.Range

Note that as dotty chose different suffix number, we don't have any assertions broken, no name clashes, as methods just become overloaded from the middle of pipeline. This is a bug, as target methods don't actually exists.

@smarter I guess that's the bug you are experiencing.
A sidenote: Current scheme seems to rely on stable order of definitions in class. This reveals problem with separate compilation of value classes, as overloading a method in a value class seems to be a binary incompatible change. This could be solved if we somehow persist reference to an appropriate extension method, eg in annotation on forwarder method.

[smarter]

Indeed, I added smarter@58f53b4 to my branch https://github.com/smarter/dotty/tree/add/value-classes which implements step 2 of SIP-15 and now most of the tests pass.