[Octave-bug-tracker] [bug #58953] Error with 'end' in index expression with nested function calls

[Fernando]

Follow-up Comment #20, bug #58953 (project octave):

I have done some tests in Matlab (2020a) to understand how expressions
involving subsref and end are processed. Maybe it can help. However, it seems
complex and I don't know if it is worth the effort to fix this issue.

So if you want to read on, I defined the following class


classdef myclass < handle
  properties
    data
  end
  methods
    function obj = myclass(data)
      obj.data = data;
    end
    function r = methodA (obj, val)
      r = val;
    end
    function r = subsref(obj,S)
      fprintf('-----------------\n')
      S.type
      S.subs
      fprintf('-----------------\n')
      r = ones(1,3);
    end
  end
end


and the following function


function r = f(x)
  r=x;
end


Then I executed:


>> obj=myclass(1:5)
obj = 
  myclass with properties:

    data: [1 2 3 4 5]
>> obj.data(end)
-----------------
ans =
    '.'
ans =
    'data'
-----------------
-----------------
ans =
    '.'
ans =
    '()'
ans =
    'data'
ans =
  1×1 cell array
    {[3]}
-----------------
ans =
     1     1     1


The subsref method is called twice. The first one evaluates obj.data, and its
result is used to call the end method. The second one evaluates the complete
expression: obj.data(3).

Another case:


>> x=1:5
x =
     1     2     3     4     5
>> obj.data(x(end))
-----------------
ans =
    '.'
ans =
    '()'
ans =
    'data'
ans =
  1×1 cell array
    {[5]}
-----------------
ans =
     1     1     1


In this case only one invocation to subsref is necessary.

Another case:


>> obj.data(idem(end))
-----------------
ans =
    '.'
ans =
    'data'
-----------------
-----------------
ans =
    '.'
ans =
    '()'
ans =
    'data'
ans =
  1×1 cell array
    {[3]}
-----------------
ans =
     1     1     1


The expression looks very similar to the previous one, but now "end" refers to
obj.data, so two subrefs are done.

It seems that Matlab tries to evaluate "idem(end)" before "obj.data", but
finds an "end" that is refering to "obj.data" and evaluates "obj.data" on the
fly.

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