Support generalized case x @ pat => in switches.

Author: sjrd

src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -339,9 +339,14 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
         case List(_: BodyTreeMaker) =>
           true
 
-        // case x =>
-        case List(_: SubstOnlyTreeMaker, _: BodyTreeMaker) =>
-          true
+        /* case x @ pat =>
+         * This includes:
+         *   case x =>
+         *   case x @ 5 =>
+         *   case x @ (5 | 6) =>
+         */
+        case (_: SubstOnlyTreeMaker) :: rest =>
+          isSwitchCase(rest)
 
         case _ =>
           false
@@ -366,12 +371,13 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
         case List(body: BodyTreeMaker) =>
           (Nil, body)
 
-        // case x =>
-        case List(subst: SubstOnlyTreeMaker, body: BodyTreeMaker) =>
-          /* Rebindings have been propagated, so `body` contains all the
-           * necessary information. `subst` can be dropped at this point.
+        // case x @ pat =>
+        case (_: SubstOnlyTreeMaker) :: rest =>
+          /* Rebindings have been propagated, so the eventual body in `rest`
+           * contains all the necessary information. The substitution can be
+           * dropped at this point.
            */
-          (Nil, body)
+          extractSwitchCase(rest)
       }
 
       def doOverlap(a: List[Int], b: List[Int]): Boolean =