Merge pull request #9555 from dotty-staging/fix-#6621

Fix #6621: Add regression test

Author: anatoliykmetyuk

tests/pos-special/fatal-warnings/i6621.scala

@@ -0,0 +1,8 @@
+object Unapply {
+  def unapply(a: Any): Option[(Int, Int)] =
+    Some((1, 2))
+}
+
+object Test {
+  val Unapply(x, y) = ""
+}