Allow pattern matching anonymous functions of arity > 1

[odersky]

This is specified in Sec. 8.5 of the SLS. Fixes #873. Review by @smarter.

[smarter]

Minor nit: it should be (x$1, ..., x$n), not (x$0, ...

[retronym]

scalac also allows pattern matching anonymous functions with an expected type of Function0:

% cat sandbox/test.scala
class Test {
  ({case _ if "".isEmpty => 0} : (() => Any)).apply()
}

%  qscalac -Xprint:typer sandbox/test.scala
[[syntax trees at end of                     typer]] // test.scala
package <empty> {
  class Test extends scala.AnyRef {
    def <init>(): Test = {
      Test.super.<init>();
      ()
    };
    ((() => () match {
  case _ if "".isEmpty() => 0
}): () => Any).apply()
  }
}

[retronym]

That might fall out of this implementation too, but the doc needs an update.

[odersky]

@retronym

need only have a FunctionN with fully defined argument types as base type.

This looks fishy. Does that not conflict with the concept of a SAM-type? What if I have

trait Sammy extends (Int => Int) {
   def apply(x: Int, y: Int): Int
   def apply(x: Int) = apply(x, x)
}

Does Sammy have one or two parameters?

[odersky]

@retronym I added support for SAM functions when adapting the arity of case lambdas. With that your test compiles as expected.

[smarter]

LGTM but I'd like @retronym to give his LGTM too :).

[retronym]

This looks fishy. Does that not conflict with the concept of a SAM-type?

I actually argued against that change. The comment thread of scala/scala#2650 is worth a read for the discussion of interactions with SAMs and the quality of error reporting.

[retronym]

I guess this comment is not applicable to dotty. It might be worth leaving a comment that contrasts the approaches, though.

[retronym]

/cc @adriaanm

[odersky]

I note the remarks but don't have the time to implement them right now. Does anyone else want to step up? If not I'll merge by tomorrow.