as initially discussed at
https://octave.discourse.group/t/assigning-classdef-objects-to-other-types-does-not-automatically-call-existing-converter-methods

A = [1:5];
B = myclass
A(4) = B

according to expected matlab behavior, Octave should first look to myclass for a method the same name as the class of A, absent that it should pass B to A's constructor to see if it can handle a myclass object, otherwise it should error.

Trying a test class:

classdef myclass
  properties
    ## no properties
  endproperties

  methods (Access = public)
    function this = myclass (varargin)
      if (nargin != 0 )
        error ("Invalid call");
      endif
    endfunction
  endmethods

  methods (Access = public)
    function retval = double (this)
      retval = NaN;
    endfunction
  endmethods
endclassdef

this should work as a converter to replace A(4) with NaN, however I instead get:

however I instead get:

>> A(4)=B
error: operator =: no conversion for assignment of 'object' to indexed 'matrix'

the only similar bug I see is an old closed bug #32651. it seems that was before classdef, but described a similar sounding issue with method precedence, where Octave "follows Mathwork's published function precedence whereas Matlab does not" preventing Octave from ever calling converter functions.

Maybe the fix here can borrow from what was learned there?