Inheriting from MPolynomialRing_libsingular crashes sage

[Etn40ff]

The following crashes sage with a SIGSEGV:

sage: from sage.rings.polynomial.multi_polynomial_libsingular import MPolynomialRing_libsingular
sage: class Foo(MPolynomialRing_libsingular):
....:     pass
sage: Foo(QQ, 2, ['x','y'], 'degrevlex')

Strangely enough this bug is triggered only when inheriting; indeed the following works as expected:

sage: from sage.rings.polynomial.multi_polynomial_libsingular import MPolynomialRing_libsingular
sage: MPolynomialRing_libsingular(QQ, 2, ['x','y'], 'degrevlex')
Multivariate Polynomial Ring in x, y over Rational Field

As it turns out this bug is due to a spurious call to __init_extra__ of Algebras(...).parent_class. Univariate polynomial rings avoid this call by setting the attribute _no_generic_basering_coercion = True; now multivariate polynomial rings do so too.

Component: algebra

Keywords: polynomial rings

Author: Salvatore Stella

Branch/Commit: 57c8182

Reviewer: Jeroen Demeyer

Issue created by migration from https://trac.sagemath.org/ticket/26958

[Etn40ff]

New commits:

d628f14

Fix 26958

[Etn40ff]

Branch: u/etn40ff/26958

[Etn40ff]

Commit: d628f14

[jdemeyer]

comment:2

Doesn't hurt I suppose...

[jdemeyer]

Reviewer: Jeroen Demeyer

[jdemeyer]

comment:3

Although maybe a minor comment about the code: can you replace

cdef public bool _no_generic_basering_coercion

by

cdef readonly _no_generic_basering_coercion

and remove the bool cimport (which serves no purpose)?

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

57c8182

Avoid importing bool

[sagetrac-git]

Changed commit from d628f14 to 57c8182

[Etn40ff]

comment:5

Done, thanks for the review.

A side note worth noting for #25558: after this patch it is possible to inherit from MPolynomialRing_libsingular but it is not possible to change its element class because the element constructor uses a function, new_MP, which hardcodes its output.

[embray]

comment:8

Retarging tickets optimistically to the next milestone. If you are responsible for this ticket (either its reporter or owner) and don't believe you are likely to complete this ticket before the next release (8.7) please retarget this ticket's milestone to sage-pending or sage-wishlist.

[vbraun]

Changed branch from u/etn40ff/26958 to 57c8182