forbid default values for extension parameters

[unkarjedy]

Compiler version

scalaVersion := "3.0.0-RC3"

Minimized code

Currently this compiles fine:

extension (x: String = "123")
  def f2 = x.reverse

Expectation

Seems that there are no use cases for it and only misleads
Probably a compiler error should be shown here.

[unkarjedy]

note that default arguments are still acceptable in using parameters

locally {
    extension (x: String)(using s: String = "default value")
      def f1: String = x.reverse
    
    "my string".f1
  }

[unkarjedy]

we may also prohibit by-name parameters in extensions,
because while even a regular argument to a by-name parameter might be surprising,
such as bar(foo()),
it's much more unexpected that foo().bar() might result in calling bar() without calling foo()

[Sporarum]

Discussing this with @mbovel, we believe we should put these checks in Refchecks
(As opposed to in the parser)

[Sporarum]

You can find our work over there:
#17321

I left some TODOs to point at what still needs to be done

[som-snytt]

I thought extension methods were just methods. I find the departure from regularity surprising, but since the issue has been discussed, there must be an obvious case of misleading that I don't see. I mean obvious but not to me.

The by-name case is the same as right-associative operators.

Edit: I meant to say, it sounds like a lint.