Type parameters should be allowed on patterns

[scabug]

unapply methods used in patterns are allowed implicit arguments. However they're not allowed type parameters, which makes this not all that useful. For example, when helping Jan Kriesten with #882 I tried the following solution:

import scala.reflect.Manifest;

object isClass{
  def unapply[T](clazz : Class[_])(implicit man : Manifest[T]) : Option[Unit] =
    if (clazz == man.erasure) Some(()) else None
}

classOf[String] match {
  case isClass[String] => "foo"
}

Gives the error

<console>:2: error: '=>' expected but '[' found.
         case isClass[String] => "foo"

The code works if you specialise T to a String instead, so the implicits are working fine, but the inability to specify a type parameter on the pattern makes this not so useful.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-884?orig=1
Reporter: @DRMacIver
See #6517, #882

[scabug]

@paulp said:
See also #2182.

[scabug]

@SethTisue said:
see also #6517

[scabug]

@SethTisue said:
scala/scala#5774