diff --git a/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala b/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
index ebaca5b626b1..a203584f27fe 100644
--- a/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
+++ b/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
@@ -116,7 +116,7 @@ object PatternMatcher {
     /** Widen type as far as necessary so that it does not refer to a pattern-
      *  generated variable.
      */
-    private def sanitize(tp: Type): Type = tp.widenExpr match {
+    private def sanitize(tp: Type): Type = tp.widenIfUnstable match {
       case tp: TermRef if refersToInternal(false, tp) => sanitize(tp.underlying)
       case tp => tp
     }
diff --git a/tests/pos/i4999.scala b/tests/pos/i4999.scala
new file mode 100644
index 000000000000..b7a856bf296a
--- /dev/null
+++ b/tests/pos/i4999.scala
@@ -0,0 +1,9 @@
+trait Foo
+final class Bar extends Foo
+
+class Test {
+  def test(xs: => Foo) = xs match {
+    case xs: Bar => 1
+    case _       => 2
+  }
+}