Test for #18345 (#18349)

Closes #18345

Author: odersky

tests/pos/i18345.scala

@@ -0,0 +1,28 @@
+extension (vec: Seq[Int])
+  def iterate[T](body: (() => Int) => T): T =
+    val iterator = vec.iterator
+    body(() => iterator.nextOption().getOrElse(0))
+
+def withSequence[T](n: Int)(body: Seq[Int] => T): T =
+  body((0 to n))
+
+def test =
+
+  withSequence(2):
+    _.iterate: next =>
+      next() + next() + next() + next()
+
+  withSequence(2):
+    _.iterate:
+      next =>
+        next() + next() + next() + next()
+
+  withSequence(2): x =>
+    x.iterate:
+      next =>
+        next() + next() + next() + next()
+
+  withSequence(2): x =>
+    x.iterate: next =>
+      next() + next() + next() + next()
+