[Octave-bug-tracker] [bug #59704] function call on classdef subobject fails with "numel: invalid return value"

[Fernando]

Follow-up Comment #2, bug #59704 (project octave):

I did some debugging. It seems that, when calling "numel" in:

https://hg.savannah.gnu.org/hgweb/octave/file/45abff4199d8/libinterp/octave-value/ov-classdef.cc#l254


octave incorrectly uses the left-value list [~, this.p] of the current
statement:
+verbose+
      [~, this.p] = bug59704_test(this.p);
-verbose-
to bind the result of the execution of numel. Because of the ~, the output of
numel is ignored, and that causes the error.

A possible fix would be to temporarily set to null the left-value list of the
current statement before calling numel. That could be done by inserting the
following lines


          octave::interpreter& interp = octave::__get_interpreter__
("octave_classdef::xnumel");
          octave::tree_evaluator& tw = interp.get_evaluator();

          octave::unwind_action act ([&tw] (const
std::list<octave::octave_lvalue> *lvl)
                            {
                              tw.set_lvalue_list (lvl);
                            }, tw.lvalue_list ());
          tw.set_lvalue_list (nullptr);


just before

https://hg.savannah.gnu.org/hgweb/octave/file/45abff4199d8/libinterp/octave-value/ov-classdef.cc#l254

You also need to include "oct-lvalue.h"

I tried it and it works, but maybe there is a better solution. I am not too
familiarized with the code.


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