register_as_coercion fails when no coercion map was already discovered

[xcaruso]

Compare:

sage: A.<a> = ZZ[]
sage: B.<b> = ZZ[]
sage: phi = A.hom([b])
sage: phi.register_as_coercion()

with:

sage: A.<a> = ZZ[]
sage: B.<b> = ZZ[]
sage: B.has_coerce_map_from(A)
False
sage: phi = A.hom([b])
sage: phi.register_as_coercion()
Traceback (most recent call last):
...
AssertionError: coercion from Univariate Polynomial Ring in a over Integer Ring to Univariate Polynomial Ring in b over Integer Ring already registered or discovered

CC: @tscrim

Component: coercion

Author: Xavier Caruso

Branch/Commit: 1b1467b

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/29517

[xcaruso]

Branch: u/caruso/register_coercion

[xcaruso]

Changed branch from u/caruso/register_coercion to none

[xcaruso]

Commit: 10ed24e

[xcaruso]

Branch: u/caruso/register_coercion

[sagetrac-git]

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

0e15a9e

better test in register_coercion

[sagetrac-git]

Changed commit from 10ed24e to 0e15a9e

[tscrim]

comment:5

Doctest failure here:

File "src/sage/categories/morphism.pyx", line 269, in sage.categories.morphism.Morphism.register_as_coercion
Failed example:
    phi.register_as_coercion()
Expected:
    Traceback (most recent call last):
    ...
    AssertionError: coercion from Univariate Polynomial Ring in x over Integer Ring to Univariate Polynomial Ring in y over Integer Ring already registered or discovered
Got:
    <BLANKLINE>

I cannot remember if this behavior was intentional or not as we might not wanted to create a coercion after explicitly checking that there was not one as a safety check. However, even if that was the case, I think this is the more natural thing to permit since the user should really know what they are doing to add this.

So once the doctest is fixed, then you can set a positive review on my behalf.

[tscrim]

Reviewer: Travis Scrimshaw

[sagetrac-git]

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

1b1467b

modify a doctest

[sagetrac-git]

Changed commit from 0e15a9e to 1b1467b