Fix TwoCohomologyGeneric when there are no conditions on the tails, leading to either an unexpected error or even a wrong output - #6477
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If no conditions on the tails were collected, the whole space of tail
vectors consists of cocycles, and `TwoCohomologyGeneric` said so by
using an identity matrix. But it sized that matrix by the number of
rewriting rules instead of by the dimension `nvars` of the space the
cocycles actually live in.
For modules of dimension 1 the two often agree, but for higher
dimensional modules they do not, and the wrongly sized matrix either
triggered the assertion in the coboundary loop
Error, SolutionMat: matrix and vector incompatible
or, if that assertion was not reached (it sits inside a conditional),
silently produced a wrong dimension for `r.cohomology`.
An example which errors before this change:
TwoCohomologyGeneric(CyclicGroup(2),
GModuleByMats([IdentityMat(2, GF(2))], 2, GF(2)));
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
fingolfin
added
kind: bug
hulpke
approved these changes
Aug 2, 2026
hulpke
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Yes, this is an oversight in a case I never tried.
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #6477 +/- ##
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Coverage 79.08% 79.09%
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Files 685 685
Lines 293736 293736
Branches 8664 8664
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+ Hits 232314 232321 +7
+ Misses 59621 59613 -8
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If no conditions on the tails were collected, the whole space of tail vectors consists of cocycles, and
TwoCohomologyGenericsaid so by using an identity matrix. But it sized that matrix by the number of rewriting rules instead of by the dimensionnvarsof the space the cocycles actually live in.For modules of dimension 1 the two often agree, but for higher dimensional modules they do not, and the wrongly sized matrix either triggered the assertion in the coboundary loop
or, if that assertion was not reached (it sits inside a conditional), silently produced a wrong dimension for
r.cohomology.An example which errors before this change: