How does numArgumen​tsFromSubs​cript work?

As an exercise, I am trying to overload the subsref method in my user-defined class myclass to support the assignment expression,

[obj.X{:}] = deal(1,2,3)

The attached code succeeds in doing so, but as the tracing output shows, numArgumentsFromSubscript is called multiple times with identical inputs before subsasgn is ever reached.

obj=myclass();
[obj.X{:}] = deal(1,2,3)

numArgumentsFromSubscript with n=3
numArgumentsFromSubscript with n=3
Executing subsasgn

obj = 
  myclass with properties:

    X: {[1]  [2]  [3]}

Why is the second invocation of numArgumentsFromSubscript necessary and, more generally, what are the formal rules for when numArgumentsFromSubscript is called relative to subsref/subsasgn?

classdef myclass

properties

X (1,3) cell

end

methods

function obj=subsasgn(obj,S,varargin)

assert(strcmp(S(1).type,'.') && ...

strcmp(S(1).subs,'X'), 'Unrecognized syntax')

obj.X=builtin('subsasgn',obj.X,S(2:end),varargin{:});

disp("Executing subsasgn") %tracing

end

function n=numArgumentsFromSubscript(obj,S,indcon)

import matlab.indexing.IndexingContext

N=numel(S);

if N>1 && isequal(S(2).subs,{':'}) && indcon==IndexingContext.Assignment

n = numel(obj.X);

else

n = 1;

end

disp("numArgumentsFromSubscript with n="+n) %tracing

end

end

end

[dpb]

Just for grins I looked at the m-code for the table incarnation and it internally sets a recursion level of two...

...
recurseAtLevel = 2;
% Perform one level of indexing, 
% then forward result to builtin numArgumentsFromSubscript