This has been resolved.  Closing report.

If you're looking into this, there is also the related bug #48682, where print_usage doesn't know that it's being called from within a method defined in a classdef block.

> it messes with the behavior of the class definition already loaded into memory.

so how does it manage that? Shouldn't it be a esad only operation? Variable name collision? Overzealous cleanup?

tested on Octave 6.2.1 (hg id: f6f664d1027a) on Windows (zip package installer)

as mentioned in bug #48682 and originating from the Octave Discourse [1],  calling print_usage() within a contructor method in a classdef class causes errors in certain subsequent calls to that class, until a clear -c is called.

Using the following test class myclass:

classdef myclass
## -*- texinfo -*-
## @deftypefn  {} {@var{A} =} myclass (@dots{})
## @end deftypefn

  properties
    A = []
  endproperties
  methods
    function this = myclass(varargin)
      if nargin == 0
        #leave empty
      elseif nargin == 1
        this.A = varargin{1};
      else
        print_usage ();
      endif
    endfunction
  endmethods
endclassdef

running through the nargin = 0, 1 and 2 cases triggers the print usage and subsequent error:

>> A = myclass(), A.A
A =

  myclass object with properties:

      A: [0x0 double]

ans = [](0x0)
>> A = myclass(1), A.A
A =

  myclass object with properties:

      A: [1x1 double]

ans = 1
>> A = myclass(3,4), A.A
error: Invalid call to myclass.  Correct usage is:

-- A = myclass (...)

Additional help for built-in functions and operators is
available in the online version of the manual.  Use the command
'doc <topic>' to search the manual index.

Help and information about Octave is also available on the WWW
at https://www.octave.org and via the help@octave.org
mailing list.
>> A = myclass(), A.A
A =

  myclass object with properties:

      A: [0x0 double]

ans = [](0x0)
>> A = myclass(1), A.A
error: class not found: myclass
error: called from
    myclass at line 14 column 16

The print_usage appears to actually execute okay (at least it displays the right help text), but for some reason it breaks subsequent attempts to assign a value to class properties. and only after a 'clear -c' does it return to normal.

adding in a breakpoint, stepping through print_usage and trying `A = myclass(1)` at each line, the "class not found" error occurs immediately after the line 62:

[text, format] = get_help_text (fullname);

as that's a compiled function I wasn't able to step through further, and I didn't see anything particularly obvious with a glance at corefcn\help.cc that would explain this.

[1] https://octave.discourse.group/t/classdef-inconsistent-testing-errors/