Fix scala#3548: set outer select type to be the type enclosing the expected type

Otherwise, ElimOuterSelect will insert `ensureConforms(tp)` in outer test,
which causes run-time exeception.

Author: liufengyun

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

@@ -770,7 +770,7 @@ object PatternMatcher {
             val expectedClass = expectedTp.dealias.classSymbol.asClass
             ExplicitOuter.ensureOuterAccessors(expectedClass)
             scrutinee.ensureConforms(expectedTp)
-              .outerSelect(1, expectedOuter.tpe.widen)
+              .outerSelect(1, expectedClass.owner.typeRef)
               .select(defn.Object_eq)
               .appliedTo(expectedOuter)
           }

tests/run/i3548.scala

@@ -0,0 +1,19 @@
+trait Common {
+  case class Data(a: String)
+}
+object O1 extends Common
+object O2 extends Common
+
+object Test {
+  def main(args: Array[String]): Unit = {
+
+    val data = O2.Data("test")
+
+    // Runtime error: java.lang.ClassCastException: O2$ cannot be cast to O1$
+    data match {
+        case O1.Data(s) => println("O1 data")
+        case O2.Data(s) => println("O2 data")
+        case _ => println("Unknown")
+    }
+  }
+}