#15303: Coercion discovery fails to be transitive
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Reporter: nbruin | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-5.13
Component: coercion | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Comment (by nbruin):
The problem is that in the present implementation, both coercions are
stored on `A`, so if a coercion from `B` to `C` is requested, it hard to
realize that `A` should even be considered.
As far as I understand, the coercion model should behave as a digraph on
the parents, where a coercion between parents exists if there is a path
from one to the other. In that model, coercion existence should be
transitive, so the behaviour described is a bug.
One way to work around it is, at least for coercion discovery, to store
the coercion always on the codomain. By #14711 this can now happen without
the implication that the codomain will keep the domain alive. We would
have to store it in a place where it can be used for coercion discovery,
though.
If it is desirable to keep the current lifetime implications for
`register_embedding` (and it probably is) then we should ensure this
separately, either by also storing the embedding map on the domain or by
just having a direct reference from the domain to the codomain.
--
Ticket URL: <http://trac.sagemath.org/ticket/15303#comment:1>
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