Add regression tests for scala#18123

Author: nicolasstucki

tests/pos/i18123.scala

@@ -0,0 +1,25 @@
+// may not compile anymore in Scala 3.4+
+package pkg
+
+trait P[+T]
+
+extension [T](inline parse0: P[T])
+  inline def | [V >: T](inline other: P[V]): P[V] = ???
+
+extension [T](inline parse0: => P[T])
+  inline def rep[V](inline min: Int = 0)(using repeater: Implicits.Repeater[T, V]): P[V] = ???
+
+object Implicits:
+  trait Repeater[-T, R]
+  object Repeater:
+    implicit def GenericRepeaterImplicit[T]: Repeater[T, Seq[T]] = ???
+
+sealed trait RegexTree
+abstract class Node extends RegexTree
+class CharClassIntersection() extends Node
+
+def classItem: P[RegexTree] = ???
+def charClassIntersection: P[CharClassIntersection] = ???
+
+def x =
+  (charClassIntersection.rep() | classItem.rep())