Allow implicit modifier in for comprehension

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Outgrowth of #1492, but that ticket has already been closed twice and the most important case works great. But as mentioned in its comments it would be useful and more consistent if this also worked:

for (implicit a <- ...) {
  ...
}

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Imported From: https://issues.scala-lang.org/browse/SI-2823?orig=1
Reporter: @paulp
See #9158, #1492
Duplicates #1305

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@odersky said:
The problem I see is the interaction with many other things. The `a' might be a pattern -- should the implicit then apply to all its variables? Then, the translation of generators in for expressions is quite involved. We'd have to speicfy how implicits are taken into account. This exceeds the design work usually associated with a ticket fix. I close the ticket but would welcome a SID that contains spec language to regulate all these things.

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@odersky said:
On second thought let's keep this open and assign to scala_community. Maybe someone picks up the ball and provides the spec enhancements.

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Kim Stebel (kimstebel) said:
Are there any guidelines on how to write such a spec?

About patterns: I think the behaviour should be analogous to

implicit val = ...

where the implicit modifier does apply to all variables in the pattern.

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@gkossakowski said:
Unassigning and rescheduling to M7 as previous deadline was missed.

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Mike Slinn (mslinn) said:
+1. It would be great to be able to write:

for { implicit x <- Some(1) } yield x

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Harold Lee (harold) said (edited on Apr 11, 2015 6:40:05 AM UTC):
I've submitted a SIP for consideration for this and I'm working on an implementation.

SIP: scala/docs.scala-lang#417

Code in progress: https://github.com/haroldl/scala/tree/sip24

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@som-snytt said:
I have a more modest PR in progress which just emits the implicit vals.

I'll also try to address #9158 by tupling extraction results instead of reapplying patterns downstream of midstream assignments. I see the SIP proposal pushes them into the body, which came up when Kim Stebel asked about it.

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@retronym said:
Link to [~apm]'s patch: https://github.com/scala/scala/pull/4730/files

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Juan Manuel Serrano (jserrano) said:
Maybe this is the right place to raise another possible extension to the implicit modifier (which I couldn't find elsewhere - sorry if it is). Namely, given, e.g.

trait MyStateMonad[F[_]] extends MonadState[F, Int]{
      import scalaz.syntax.monad._
      implicit val _ = this // necessary in order to use the syntactic sugar
      ...
}

it'd be nice to be able to write that just as follows:

trait MyStateMonad[F[_]] extends MonadState[F, Int]{ implicit this =>
      import scalaz.syntax.monad._
      ...
}