class C
object C:
  extension (c: C) def equals(that: Any): Boolean = false // warn

object X:
  class C
  opaque type D <: C = C
  object D:
    extension (d: D) def equals(that: Any): Boolean = false // nowarn

-- [E194] Potential Issue Warning: .../no-extension.scala:12:23 -
12 |  extension (c: C) def equals(that: Any): Boolean = false
   |                       ^
   |Extension method equals will never be selected
   |because C already has a member with the same name and compatible parameter types.