OK, so it makes sense to limit the types of expressions that may appear inside the list of attributes.

RE: Comment #12.  Currently, Octave parses an attribute specification as one of

identifier
identfier '=' expression
EXPR_NOT identifier

Unless there is something else that should be allowed, that probably doesn't need to change, unless Matlab allows things like

methods (some + arbitrary + expression)
end

without error or warning.

Either way, validating the attribute expression can be deferred until the classdef object is finally constructed.  It doesn't need to happen when the syntax is initially parsed.

In addition singular meta-classes seem to work:

classdef testclassdef

  methods (Access = public)
    function this = testclassdef (varargin)
      a = 1;
    end
  end

  methods (Access = {?testclassdef2, ?testclassdef3})
    function this = say_hello(this)
      disp('hello');
    end
  end

  methods (Access = ?testclassdef2)
    function this = say_hello2(this)
      disp('hello2');
    end
  end

end

So, it looks like the following is allowed for the `Access` attribute:
- any of the keywords `public`, `private`, `protected`
- string literals corresponding to these keywords: `'public'`, `'private'`, `'protected'`
- single meta-class objects (like `?testclassdef2`)
- cell arrays of meta-class objects (like `{?testclassdef2, ?testclassdef3}`)

Everything else seems to result in an error on construction (if the methods-block is not empty).

just tried it.  get the :

Error using testclassdef
Error: File: testclassdef.m Line: 9 Column: 21
'Access' must be public, protected, private or a list of classes.  For more information,
see Specify Access to Class Members.

curiously, matlab doesn't error if the methods block is empty. seems to ignore the block altogether.

@Markus: The "Access = hidden" syntax is not correct.  But, you could answer my question in comment #5 by creating an m-file "return_public.m" which has the following code:

function retval = return_public ()
  retval = 'public';
end

and then changing the code in testclassdef.m to

methods (Access = return_public ())

Does Matlab error for this code which would state that only string literals which are constants at compile-time are allowed?

I tested with the following class in Matlab R2021a:

classdef testclassdef

  methods (Access = public)
    function this = testclassdef (varargin)
      a = 1;
    end
  end

  methods (Access = hidden)
    function this = say_hello(this)
      disp('hello');
    end
  end

end

An object of that class can be created. But an error is thrown when trying to access that method:

>> t = testclassdef
Error using testclassdef
Error: File: testclassdef.m Line: 9 Column: 21
'Access' must be public, protected, private or a list of classes.  For more information, see Specify Access to Class Members.

Btw, the error message in Matlab could be more specific. Afaict, `Access` can be set to `public`, `protected`, `private`, or a cell array of meta-classes (like `{?testclassdef2}`).

whoops. 

should this be closed, or is there anything about syntax errors triggering a figure window instead of an error message worth looking into?

that seems to make sense.  just tried in another class adding Access = hidden to the properties block and sure enough it pops up a figure window.

Tested on Windows in Octave 6.4.0 and 7.0.1 (hg id: 366aa563dd2e)

creating a classdef class, I started getting an odd empty figure window to pop up when creating the object. after some trial and error, I found that having a methods section with (Access = hidden) appears to be the trigger. it seems to not matted what populates the methods, properties, etc.

testclassdef.m:

classdef testclassdef

  methods (Access = public)
    function this = testclassdef (varargin)
      a = 1
    endfunction
  endmethods

#  methods (Access = hidden)
#  endmethods

endclassdef

>> close all;clear all;
>> testclassdef
a = 1
ans =

  testclassdef object with properties:

uncomment the hidden methods section

>> close all;clear all;
>> testclassdef
**empty figure window appears**
a = 1
ans =

  testclassdef object with properties: