if _sub_ is not defined, then the coercion framework enters an infinite loop

[saliola]

sage: C = cartesian_product([ZZ, ZZ])                                                                                                                                     
sage: C
The cartesian product of (Integer Ring, Integer Ring)
sage: c = C.an_element()
sage: c
(1, 1)
sage: hasattr(c, '_sub_')
False
sage: c - c
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
...
RuntimeError: maximum recursion depth exceeded in __instancecheck__

implementing a _sub_ method fixes the problem (such a method will be implemented for Cartesian products on a separate ticket).

CC: @nthiery

Component: coercion

Keywords: days64

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/17991

[videlec]

comment:1

This is fixed in #18275 (closed)

[tscrim]

Reviewer: Travis Scrimshaw