Fix #4315: handle local child from scalac

Author: liufengyun

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

@@ -497,6 +497,8 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
   def refine(parent: Type, child: Symbol): Type = {
     if (child.isTerm && child.is(Case, butNot = Module)) return child.termRef // enum vals always match
 
+    if (child.name == tpnme.LOCAL_CHILD) return child.typeRef
+
     val childTp = if (child.isTerm) child.termRef else child.typeRef
 
     val resTp = instantiate(childTp, parent)(ctx.fresh.setNewTyperState())

tests/patmat/i4315.scala

@@ -0,0 +1,8 @@
+import scala.concurrent.duration.{Duration, FiniteDuration}
+
+class Test {
+  def test(d: Duration) = d match {
+    case finite: FiniteDuration =>
+    case d =>
+  }
+}