diff --git a/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala b/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
index 41d21d61cd12..e6886b99e431 100644
--- a/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
+++ b/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
@@ -177,12 +177,19 @@ class TreeChecker extends Phase with SymTransformer {
       res
     }
 
+    def withPatSyms[T](syms: List[Symbol])(op: => T)(implicit ctx: Context) = {
+      nowDefinedSyms ++= syms
+      val res = op
+      nowDefinedSyms --= syms
+      res
+    }
+
     def assertDefined(tree: untpd.Tree)(implicit ctx: Context) =
       if (
         tree.symbol.maybeOwner.isTerm &&
         !(tree.symbol.is(Label) && !tree.symbol.owner.isClass && ctx.phase.labelsReordered) // labeldefs breaks scoping
       )
-        assert(nowDefinedSyms contains tree.symbol, i"undefined symbol ${tree.symbol}")
+        assert(nowDefinedSyms contains tree.symbol, i"undefined symbol ${tree.symbol} at line " + tree.pos.line)
 
     /** assert Java classes are not used as objects */
     def assertIdentNotJavaClass(tree: Tree)(implicit ctx: Context): Unit = tree match {
@@ -400,7 +407,7 @@ class TreeChecker extends Phase with SymTransformer {
       }
 
     override def typedCase(tree: untpd.CaseDef, pt: Type, selType: Type, gadtSyms: Set[Symbol])(implicit ctx: Context): CaseDef = {
-      withDefinedSyms(tree.pat.asInstanceOf[tpd.Tree].filterSubTrees(_.isInstanceOf[ast.Trees.Bind[_]])) {
+      withPatSyms(tpd.patVars(tree.pat.asInstanceOf[tpd.Tree])) {
         super.typedCase(tree, pt, selType, gadtSyms)
       }
     }
diff --git a/tests/pos/i4198.scala b/tests/pos/i4198.scala
new file mode 100644
index 000000000000..667bd983fc9e
--- /dev/null
+++ b/tests/pos/i4198.scala
@@ -0,0 +1,15 @@
+class Foo {
+  def foo(x: Any): Unit = {
+    x match {
+      case y: Bar[_] =>
+        y.value match {
+          case value: Bar[_] => // here x is an instance of Bar[Bar[_]]
+          case _ =>
+        }
+    }
+  }
+}
+
+class Bar[T] {
+  def value: T = ???
+}