test: add in a regression for scala#12679

[skip community_build]

closes scala#12679

Author: ckipp01

tests/pos/i12679.scala

@@ -0,0 +1,9 @@
+// https://github.com/lampepfl/dotty/issues/12679
+
+object Example:
+  def foo[F[_]](qux: String, quux: String = ""): F[Unit] = ???
+
+  def foo[F[_]](qux: Boolean): F[Unit] = ???
+
+  def example[F[_]](maybeQux: Option[String], bool: Boolean) =
+    maybeQux.fold(foo[F](bool))(foo[F](_))