regression in pattern viability checking

[scabug]

Correctly errors out in 2.9, compiles without warning in 2.10.

object Test {
  sealed trait Foo
  object Bar extends Foo

  def f(x: Foo) = x match {
    case scala.Right => true
    case _           => false
  }
}
// scala 2.9
// ./a.scala:9: error: pattern type is incompatible with expected type;
//  found   : object Right
//  required: Test.Foo
//     case scala.Right => true
//                ^
// one error found

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-7655?orig=1
Reporter: @paulp
Affected Versions: 2.9.3, 2.10.0
See #7211

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@retronym said:
The change wasn't in the typechecker, it was in the library. Matching against scala.util.Right still gives the error.

object Test {
  sealed trait Foo
  object Bar extends Foo

  object Other
  val OtherAlias = Other

  def f(x: Foo) = x match {
    case Other => true // warns
    case _     => false
  }

  def g(x: Foo) = x match {
    case OtherAlias => true // no warning.
    case _          => false
  }
}

The check happens in adapt, which calls inferModulePattern if the pattern's symbol is a module. Seems to me that we should make the check for any stable identifier pattern:

A stable identifier pattern is a stable identifier r (§3.1). The type of r must conform to the expected type of the pattern. The pattern matches any value v such that r == v (§12.1).

[scabug]

@retronym said:
scala/scala#2742

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@retronym said:
Related to #7211

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@retronym said:
Turns out this interpretation of the spec is likely to break a fair amount of code. I've updated the PR with the changes to our distro needed after bootstrapping with this change. Is the spec right?

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@paulp said:
See also #785 (which you noted relates to #7456, which in turn you noted relates to #7211, which above you noted relates to this ticket, so I guess I'm just adding an edge to the graph.)

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@adriaanm said:
sounds like a won't fix...?

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@adriaanm said:
Should clarify the spec and perhaps provide a lint check.