Fix bootstrapping

This corner case optimization is unsafe and no code depends on it.

Author: liufengyun

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

@@ -255,8 +255,6 @@ trait SpaceLogic {
            Empty
         else if (canDecompose(tp2))
           tryDecompose2(tp2)
-        else if (isSubType(tp2, tp1) &&covers(fun, tp2))
-          minus(a, Prod(tp1, fun, signature(fun, tp1, ss.length).map(Typ(_, false))))
         else
           a
       case (Prod(tp1, fun1, ss1), Prod(tp2, fun2, ss2)) =>

tests/patmat/dotty-trees.scala

@@ -0,0 +1,11 @@
+abstract class Tree[-T >: Null]
+
+case class TypeTree[-T >: Null]() extends Tree[T]
+
+abstract class DerivedTypeTree() extends TypeTree[Null]
+
+def foo(tree: Tree[Null]): Unit =
+  tree match
+  case _: DerivedTypeTree =>
+  case TypeTree() =>
+  case _ =>