How do I encapsulate an old-style class with a classdef class?

I'm re-writing one of the objects in our obj = class(obj,<blah>); hierarchy to classdef syntax. But I only want to rehash ONE class (we have 26, and some of them are... 'of notable size'). The class up for conversion "has a" relationship with two other classes still defined in the old syntax.

The situation looks like this:

Contents of +newfangled\zippy.m:

classdef zippy
    % ZIPPY nifty shiny
    properties
          % elegant 'has-a' syntax, but validate engine won't shut up.
          spam(5,1) grumpy 
    end % public properties
end % classdef 

>> eggs = zippy
Error defining property 'spam' of class 'newfangled.zippy':
Class named 'grumpy' is undefined or does not support property validation.

>> which grumpy
C:\<blah>\<yadda>\<etc>\@grumpy\grumpy.m

Which contains roughly:

function obj = grumpy(varargin)
    switch (nargin)
        case 0
             obj.noneyer = true;
             obj.comments = 'business.';
             obj = class(obj,'grumpy');
        case 1
              obj = digSumptinUp(varargin{1})
        otherwise
              error('Harumph.');
        end % blade
end % classy grumpy constructor

Is there a way to get the classdef 'validation engine' to %#<INVLDTYPEOK> a parameter?

>> lasterror
ans = 

    struct with fields:

         message: 'Error defining property 'grumpy' of class 'newfangled.zippy':...'
      identifier: 'MATLAB:class:InvalidType'
           stack: [0x1 struct]

So it has a name--but how do I wrap a default constructor in an internal try-catch block to shove 'MATLAB:class:InvalidType' down a hole?

So I tried:

classdef zippy
     % ZIPPY do dah
     properties
         % type defaults to double
         spam
     end % public properties
     methods
         function obj = zippy(varargin)
               spam = grumpy;  % late-bind hack-- but won't coerce spam.
         end % constructor
      end % public methods
end % classdef

>> eggs = zippy
The following error occurred converting from grumpy to double:
Conversion to double from grumpy is not possible.

And sure, coercion is a RHS thing. But is there a way to re-assign the parameter class late, or create the parameter entirely in the constructor (like the old syntax does)?

Oh wait-- can I...

classdef zippy
     % ZIPPY funcy
     properties
         % anonymous pass-through.  Parameter is a function handle.
         spam @(ham) grumpy (ham)
     end % public properties
end % classdef

>> effs = zippy
Unexpected MATLAB operator

...because grumpy isn't really a function, it's a class. But you can see where I'm going with this. Can I even make a parameter of type function_handle?

classdef zippy
     % ZIPPY funcy
     properties
         % anonymous pass-through.  Parameter is a function handle.
         spam function_handle
     end % public properties
end % classdef

>> effg = zippy
Error defining property 'spam' of class 'newfangled.zippy':
Unable to construct default object of class function_handle.

Again: the validator won't shut up.

>> lasterror

struct with fields:

       message: 'Error defining property 'spam' of class 'newfangled.zippy':...'
    identifier: 'MATLAB:class:DefaultPropertyValueRequired'
         stack: [0x1 struct]

And again: it has an identifier-- but how do I catch it?

So...what's the trick?

[Bob Clark]

So then I tried:

classdef zippy
    % ZIPPY anon
    properties
        spam
    end % properties
    methods
    function obj = zippy(varargin)
        spam = grumpy;
    end % constructor
end % classdef

>> ehhi = zippy
Error using class
The name 'grumpy' is not a valid class name.
Error in newfangled.zippy (line <hash>)
     obj = class(obj,'grumpy');

But 'grumpy' is just a string. How can it be an invalid class name? What gives?