Fix scala#2378: add tests and address review feedback

Author: liufengyun

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

@@ -354,6 +354,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
     case Bind(_, pat) => project(pat)
     case UnApply(_, _, pats) =>
       if (pat.tpe.classSymbol.is(CaseClass))
+        // FIXME: why dealias is needed here?
         Kon(pat.tpe.stripAnnots.dealias, pats.map(pat => project(pat, roundUp)))
       else if (roundUp) Typ(pat.tpe.stripAnnots, false)
       else Empty

tests/pos/i2378.scala

@@ -0,0 +1,29 @@
+trait Cap
+
+trait Toolbox {
+  type Tree
+
+  val tpd: TypedTrees
+  trait TypedTrees {
+    type Tree
+  }
+
+  val Apply: ApplyImpl
+  trait ApplyImpl {
+    def unapply(tree: Tree): Option[(Tree, Seq[Tree])]
+    def unapply(tree: tpd.Tree)(implicit c: Cap): Option[(tpd.Tree, Seq[tpd.Tree])]
+  }
+}
+
+class Test(val tb: Toolbox) {
+  import tb._
+  implicit val cap: Cap = null
+
+  def foo(tree: Tree): Int = tree match {
+    case Apply(fun, args) => 3
+  }
+
+  def bar(tree: tpd.Tree): Int = tree match {
+    case Apply(fun, args) => 3
+  }
+}