Fix false warning about unreachable code in Dotty

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -509,14 +509,14 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
           case space => List(space)
         }
       case OrType(tp1, tp2) => List(Typ(tp1, true), Typ(tp2, true))
-      case _ if tp =:= ctx.definitions.BooleanType =>
+      case tp if tp =:= ctx.definitions.BooleanType =>
         List(
           Typ(ConstantType(Constant(true)), true),
           Typ(ConstantType(Constant(false)), true)
         )
-      case _ if tp.classSymbol.is(Enum) =>
+      case tp if tp.classSymbol.is(Enum) =>
         children.map(sym => Typ(sym.termRef, true))
-      case _ =>
+      case tp =>
         val parts = children.map { sym =>
           if (sym.is(ModuleClass))
             refine(tp, sym.sourceModule.termRef)

tests/patmat/dotty-unreachable.scala

@@ -0,0 +1,17 @@
+object Types {
+  abstract case class TermRef(val prefix: String, name: String) {
+    type ThisType = TermRef
+
+    def alts: List[TermRef] = ???
+  }
+}
+
+class Test {
+  def foo(tp: Types.TermRef): Unit = {
+      tp.alts.filter(_.name == "apply") match {
+        case Nil =>
+        case alt :: Nil =>
+        case alt =>
+      }
+  }
+}