Unreachable code check in partial functions needs improvement

[scabug]

Partial functions don't appear to be checked for unreachable code to the same extent as match expressions (the trivial case with a variable pattern is checked properly):

Some(1) match { 
  case Some(a) => 2
  case Some(b) => 3 // warning: "unreachable code"
}

Some(Some(1)) collect { 
  case Some(a) => 2
  case Some(b) => 3 // no warning
}

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-6902?orig=1
Reporter: Rob Norris (rnorris)
Affected Versions: 2.10.0-RC5

[scabug]

@retronym said:
scala/scala#1928