Merge pull request #9550 from dotty-staging/fix-#9248

Fix #9248: Add test

Author: nicolasstucki

tests/pos/i9248.scala

@@ -0,0 +1,8 @@
+class C(var a: Int)
+
+extension (c: C)
+  def foo: Int = c.a
+  def foo_=(a: Int): Unit = c.a = a
+
+val c = C(10)
+val test = c.foo