td04ad with rowcol = 'C' fails in 4 out of 5 testcases

[Peter3579]

When compiling slycot on NixOS (Linux), 4 out of 5 "td04ad_c" testcases fail. Ignoring this (and nevertheless installing slycot) leads to 170 failing testcases in the control package.

Both, v0.6.0 and master, show the same issue. I used SLICOT v5.9.

=========================== short test summary info ============================
FAILED ../../nix/store/4i61izh6bxdjz498fq28j2149dh5m2bk-python3.13-slycot-0.6.0/lib/python3.13/site-packages/slycot/tests/test_td04ad.py::test_staticgain - slycot.exceptions.SlycotParameterError: The following argument had an illeg...
FAILED ../../nix/store/4i61izh6bxdjz498fq28j2149dh5m2bk-python3.13-slycot-0.6.0/lib/python3.13/site-packages/slycot/tests/test_td04ad.py::test_td04ad_static - slycot.exceptions.SlycotParameterError: The following argument had an illeg...
FAILED ../../nix/store/4i61izh6bxdjz498fq28j2149dh5m2bk-python3.13-slycot-0.6.0/lib/python3.13/site-packages/slycot/tests/test_td04ad.py::test_mixfeedthrough - slycot.exceptions.SlycotParameterError: The following argument had an illeg...
FAILED ../../nix/store/4i61izh6bxdjz498fq28j2149dh5m2bk-python3.13-slycot-0.6.0/lib/python3.13/site-packages/slycot/tests/test_td04ad.py::test_tfm2ss_6 - slycot.exceptions.SlycotParameterError: The following argument had an illeg...
================== 4 failed, 132 passed, 2 warnings in 1.51s ===================

All these tests fail with:
E slycot.exceptions.SlycotParameterError: The following argument had an illegal value: dcoeff

[roryyorke]

We build against a custom version of SLICOT, currently https://github.com/python-control/SLICOT-Reference/tree/slycot-changes . This is mentioned in our README: https://github.com/python-control/Slycot?tab=readme-ov-file#get-it-from-github-archives , but we could be clearer that this isn't vanilla upstream SLICOT.

Our changes include diffs to TD04AD.f. I don't see anything in SLICOT issues about this, assume we never filed an issue with them --- I'll make a note to do so.

[Peter3579]

Thanks a lot for pointing out that a patched version of SLICOT-Reference is needed. Using rev= "795051cbc2a1d4766753e9ab3bac13eaf731f8d6"; did the trick. The tests pass now.