Loosen "only one overload can define defaults" restriction

[scabug]

If we encode the arity of the method into the default getter name, we don't have something completely general, but we would eliminate most of the pain.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-8161?orig=1 Reporter: @retronym Affected Versions: 2.12.0-RC1

[scabug]

@retronym said: https://github.com/retronym/scala/compare/ticket;8161?expand=1

[scabug]

@nilskp said: You could go further than that and encode the argument types, using 1 char letters.

[SethTisue]

in short, it makes sense to disallow this:

object O1 {
  def foo(y: String = "bar") = ()
  def foo(y: Int = 3) = ()
}

since foo() is ambiguous, but it seems like this ought to be allowed:

object O2 {
  def foo(x: String, y: String = "bar") = ()
  def foo(x: Int, y: Int = 3) = ()
}

but the compiler rejects it with "in object O2, multiple overloaded alternatives of method foo define default arguments"

https://stackoverflow.com/questions/4652095/why-does-the-scala-compiler-disallow-overloaded-methods-with-default-arguments explains what the problem is. In short, in the current encoding the compiler would need to emit two different zero-arity methods named foo$default$2

The SO answers include suggested workarounds.

The restriction exists in Dotty as well, for interoperability with Scala 2.