I verified Octave works with minimal example from comment #5.

Marking bug as Fixed and closing report.

Hi Markus, your minimal example is perfect.

I think your suggestion of querying nargout would be superior to the try-catch-construct because it does not catch other errors and helps keeping code simple. The problem I see is that throwing the exception inside the validation function would not allow to display additional information in inputParser about the reasons.

Your suggested change of inputParser looks good so far. I would recommend testing the the error id of the exception to distinguish between "too few output values" and any implementation or runtime error inside the validation function. You may have a look on my comment #2 for this.

I try to get closer to the core of this error. I use a function dirPlus() from matlab fileexchange, that specifies validateattributes() as a validation function to input parser:

    addRequired(parser, 'rootPath', ...
                @(s) validateattributes(s, {'char'},{'nonempty'}));

Earlier versions of input parser specified that the validation function needs to return something on error and nothing if ok. This is why using validateattributes() was ok in this usage but now is not any more.

If there is a problem in the 7.1.0 it has been introduced to validateattributes() some versions ago. Nevertheless it should not produce any problem if validateattributes() just returns true in all cases? This would be an option to fix my troubles at least...

Sorry Markus, I had an outdated version of inputParser in my path. After removing the old version the problem essentially stays the same, just the code looks exactly as you quoted it.

It fails when executing val (in) because validatedattributes() that is provided as function handle in val is not returning a result if argument check is ok:

error: validateattributes: function called with too many outputs
error: called from
    validateattributes

After I rewrote the validate_arg() function like this it is behaving as expected:

    function validate_arg (this, name, val, in)

      try
        check = val(in);
      catch
        myerror = lasterror();
        if strcmpi(myerror.identifier,"Octave:invalid-fun-call")
          ## check is ok if no result
          check = true;
        else
          ## rethrow error
          disp(myerror());
          error(myerror.identifier,myerror.message);
        endif
      end_try_catch

      if (!check)
        this.error (sprintf ("failed validation of %s with %s",
                             toupper (name), func2str (val)));
      endif
      this.Results.(name) = in;

    endfunction

I can't tell if it would be better to modify validateattributes()

The validate_arg function as part inputParser class fails in octave 7.1.0 at the marked position while it worked up to 6.4

    function validate_arg (this, name, val, in)
            % Check validation result to be empty in case of the use of validateattributes function
            % An empty result will be interpreted as valid arguments
    -->        check = {val(in)};
            if (~isempty(check) && ~val(in))
              this.error (sprintf ("failed validation of %s with %s", toupper (name),func2str(val)));
            endif
            this.Results.(name) = in;
        endfunction

The reason is that val() is a function handle for validateattributes() that checks the validity of the in parameter and does only return a result if the test fails. If everything is ok it does not return a result. Due to changes in octave 7.1 interpreter this is now handled as an error (see bug #62418).

There may be more functions affected by this change.