Add support for ignoring function arguments

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When a method of a specific signature needs to be implemented, it would be helpful to somehow mark the arguments that are not going to be used.

Currently, the best solution is that we assign an argument name that hints that it is obviously not going to be used.

def render(dontUseMe: NodeSeq) = {
  ...
}

I propose that we add a yet additional usage of the underscore, this time for ignoring a method argument.

def render(_: NodeSeq) = {
  ...
}

This should fit in nicely with the already existing method of ignoring the argument when creating a function value:

val render = (_: NodeSeq) => {
  ...
}

It is also compatible (in spirit) with the pattern matching system for ignoring everything but the type of the argument.

case _: NodeSeq =>
  ...

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-5449?orig=1
Reporter: @melezov
Affected Versions: 2.10.0

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@adriaanm said:
Assigning to community for a more detailed proposal, discussion on scala-internals, etc.

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Shelby Moore III (shelby) said:
Bump. There is more than minor. Self-documenting code is very important. Why should the reader of the code have to go searching through the entire function body to realize the function parameter is not used.

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@melezov said:
I've especially found this to be bothersome when building DSLs which pass in objects, such as:

    object FROM
    
    object SELECT {
      def *(f: FROM.type) = ???
    }

One could try and use the function value, but this will not work if we need polymorphism...

      val * = (_: FROM.type) => ???
      val * = (_: TO.type) => ??? // clash

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@som-snytt said:
Also typelevel/scala#9