This bug report is for Squeak 3.7 alpha latest update: 5623

When you create a subclass, you cannot change it later
into a variable subclass.

To see what goes wrong, please try this:

1. Open a class browser and select class Time.
2. Replace the class name 'Time' with 'TestClass' to
  create a subclass of Magnitude. Select 'accept' from
  the view menu to create the subclass.
3. Replace the 'subclass:' with 'variableSubclass:'
  and 'accept' again to change the just created subclass
  into a variable subclass. This should be possible because
  'TestClass' has no subclasses at all (subclasses
   impose restrictions to the possible type of a class)
   and because Magnitude is a fixed-sized class. 

You will get a error notifier saying:
'TestClass cannot be compiled'.

The problem is, as far as I can see, in the
method ClassBuilder>>validateSubclassFormat:from: forSuper:extra:

Here we have a check that reads:

oldClass withAllSubclassesDo:
    [:sub |   <omission>
        "If we get this far, check whether the immediate
         subclasses of oldClass can keep its layout."
         (newType ~~ #normal) 
             ifTrue:
               [ self validateSubclass: sub 
                        canKeepLayoutFrom: oldClass
                        forSubclassFormat: newType
     ]        ].

The troublemaker is the  "withAllSubclassesDo:".
This method answers a collection that contains
*the receiver class* and all its subclasses.
Therefore, for one evaluation of 
   'self  validateSubclass: ...'
the identity  sub == oldClass  holds, and this
is clearly not intended.
For the test, we need a collection that contains
only the subclasses of the receiver class, but not
the receiver class itself. Such a collection is
obtained with method  'allSubclassesDo:'.

Attached you find a proposal for a fix. I think that it
will not cause problems elsewhere but your advice
is very welcome.

Greetings, Boris 
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