SI-6195 stable members can only be overridden by stable members

1 files changed, 12 insertions, 4 deletions

diff --git a/07-classes-and-objects.md b/07-classes-and-objects.md
index 139599b7ef..012bbe44ff 100644
--- a/07-classes-and-objects.md
+++ b/07-classes-and-objects.md
@@ -350,16 +350,24 @@ $M'$:
 - If $M$ and $M'$ are both concrete value definitions, then either none
   of them is marked `lazy` or both must be marked `lazy`.
 
-A special rule concerns parameterless methods. If a paramterless
-method defined as `def $f$: $T$ = ...` or `def $f$ = ...` overrides a method of
-type $()T'$ which has an empty parameter list, then $f$ is also
-assumed to have an empty parameter list.
+A stable member can only be overridden by a stable member.
+For example, this is not allowed:
+
+```
+class X { val stable = 1}
+class Y extends X { override var stable = 1 } // error
+```
 
 Another restriction applies to abstract type members: An abstract type
 member with a [volatile type](#volatile-types) as its upper
 bound may not override an abstract type member which does not have a
 volatile upper bound.
 
+A special rule concerns parameterless methods. If a parameterless
+method defined as `def $f$: $T$ = ...` or `def $f$ = ...` overrides a method of
+type $()T'$ which has an empty parameter list, then $f$ is also
+assumed to have an empty parameter list.
+
 An overriding method inherits all default arguments from the definition
 in the superclass. By specifying default arguments in the overriding method
 it is possible to add new defaults (if the corresponding parameter in the