[Octave-bug-tracker] [bug #34173] Unary negative casts permutation matrices into full matrices

[Nicholas Jankowski]

Follow-up Comment #18, bug #34173 (project octave):

Yeah I didn't even know Octave used a sparse permutation type for memory
efficiency.

I guess the whole bug thread suggests three options if this bug were to be
"fixed"

1 preserve the perm. matrix and see if the reported bug can be fixed, where
unary neg turns a perm into a full matrix.  it does seem odd for that
seemingly simple operation to trigger expansion. This was attempted in comment
#10 but reverted by Rik in comment #12 because bug #44316 found it was causing
incorrect  comparison results, which obviously takes precedence over
efficiency. Seems this fix would really require revisiting that bug solution.

2 comment #13 suggested leaving it alone, documenting somewhere not to rely on
the memory efficiency of the permutation type and/or list the arithmetic and
comparison ops that convert back to using the full memory requirement.

3 comment #16 hinted at doing away with permutation (and maybe othrr") memory
saving data types to simplify the codebase and avoid problems like this
report. I believe this was separately discussed for ranges not too long ago.

4 (implied) leave it as a won't fix. It's a 9 year old bug that isn't exactly
reporting something broken, just unexpectedly imposing the "normal" memory
requirement.

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