Merge pull request #12894 from dotty-staging/fix-7325

Fix #7325: Add test case.

Author: smarter

tests/neg/i7325.scala

@@ -0,0 +1,9 @@
+import scala.collection._
+
+class MySeq[A](private val underlying: Seq[A]) // error
+  extends Seq[A]
+  with SeqOps[A, MySeq, MySeq[A]] {
+  def apply(n: Int) = underlying(n)
+  def iterator = underlying.iterator
+  def length = underlying.size
+}