Use the upper bound of type skolems to enumerate subtypes

src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala

@@ -88,6 +88,7 @@ trait TreeAndTypeAnalysis extends Debugging {
       case sym if sym.isRefinementClass => enumerateRefinement(tp, grouped)
       case sym if sym.isSealed          => enumerateSealed(tp, grouped)
       case sym if sym.isCase            => List(List(tp))
+      case sym if sym.isTypeSkolem      => enumerateSubtypes(sym.info.upperBound, grouped) // pos/t12277
       case sym                          => debug.patmatResult(s"enum unsealed tp=$tp sym=$sym")(Nil)
     }
 

test/files/pos/t12277.scala

@@ -0,0 +1,12 @@
+// scalac: -Xlint:strict-unsealed-patmat -Werror
+sealed trait A
+final case class B() extends A
+final case class C() extends A
+
+object x extends App {
+
+  def matcher[A1 <: A](a1: A1) = a1 match {
+    case x @ (_: B | _: C) => println("B")
+  }
+
+}

test/files/run/patmat-behavior.check

@@ -136,6 +136,30 @@ patmat-behavior.scala:55: warning: match may not be exhaustive.
 It would fail on the following inputs: C00(), C01(_), C10(_), C11(_, _), C20(_, _), C21(_, _, _)
     def gc6[A](x: C[A]) = x match { case F00() => ??? ; case F10(x) => x ; case F20(x, y) => x ; case F01(xs @ _*) => xs.head ; case F11(x, ys @ _*) => x ; case F21(x, y, zs @ _*) => x }
                           ^
+patmat-behavior.scala:57: warning: match may not be exhaustive.
+It would fail on the following inputs: C00(), C01(_), C10(_), C11(_, _), C20(_, _), C21(_, _, _)
+    def gd1[A, B <: C[A]](x: B) = x match { case F00() => ??? ; case F10(x) => x ; case F20(x, y) => x ; case F01(xs @ _*) => xs.head ; case F11(x, ys @ _*) => x ; case F21(x, y, zs @ _*) => x }
+                                  ^
+patmat-behavior.scala:58: warning: match may not be exhaustive.
+It would fail on the following inputs: C00(), C01(_), C10(_), C11(_, _), C20(_, _), C21(_, _, _)
+    def gd2[A, B <: C[A]](x: B) = x match { case F00() => ??? ; case F10(x) => x ; case F20(x, y) => x ; case F01(xs @ _*) => xs.head ; case F11(x, ys @ _*) => x ; case F21(x, y, zs @ _*) => x }
+                                  ^
+patmat-behavior.scala:59: warning: match may not be exhaustive.
+It would fail on the following inputs: C00(), C01(_), C10(_), C11(_, _), C20(_, _), C21(_, _, _)
+    def gd3[A, B <: C[A]](x: B) = x match { case F00() => ??? ; case F10(x) => x ; case F20(x, y) => x ; case F01(xs @ _*) => xs.head ; case F11(x, ys @ _*) => x ; case F21(x, y, zs @ _*) => x }
+                                  ^
+patmat-behavior.scala:60: warning: match may not be exhaustive.
+It would fail on the following inputs: C00(), C01(_), C10(_), C11(_, _), C20(_, _), C21(_, _, _)
+    def gd4[A, B <: C[A]](x: B) = x match { case F00() => ??? ; case F10(x) => x ; case F20(x, y) => x ; case F01(xs @ _*) => xs.head ; case F11(x, ys @ _*) => x ; case F21(x, y, zs @ _*) => x }
+                                  ^
+patmat-behavior.scala:61: warning: match may not be exhaustive.
+It would fail on the following inputs: C00(), C01(_), C10(_), C11(_, _), C20(_, _), C21(_, _, _)
+    def gd5[A, B <: C[A]](x: B) = x match { case F00() => ??? ; case F10(x) => x ; case F20(x, y) => x ; case F01(xs @ _*) => xs.head ; case F11(x, ys @ _*) => x ; case F21(x, y, zs @ _*) => x }
+                                  ^
+patmat-behavior.scala:62: warning: match may not be exhaustive.
+It would fail on the following inputs: C00(), C01(_), C10(_), C11(_, _), C20(_, _), C21(_, _, _)
+    def gd6[A, B <: C[A]](x: B) = x match { case F00() => ??? ; case F10(x) => x ; case F20(x, y) => x ; case F01(xs @ _*) => xs.head ; case F11(x, ys @ _*) => x ; case F21(x, y, zs @ _*) => x }
+                                  ^
 patmat-behavior.scala:68: warning: match may not be exhaustive.
 It would fail on the following input: C00()
     def gb1[A](x: C00[A]) = x match { case E00() => ??? ; case E10(x) => x ; case E20(x, y) => x ; case E01(xs @ _*) => xs.head ; case E11(x, ys @ _*) => x ; case E21(x, y, zs @ _*) => x }