#15345: Laurent polynomial rings don't preserve coercion
-------------------------+-------------------------------------------------
   Reporter:  darij      |            Owner:
       Type:  defect     |           Status:  new
   Priority:  major      |        Milestone:  sage-5.13
  Component:  algebra    |         Keywords:  laurent polynomials,
  Merged in:             |  polynomials, coercion, categories
  Reviewers:             |          Authors:
Work issues:             |  Report Upstream:  N/A
     Commit:             |           Branch:
   Stopgaps:             |     Dependencies:
-------------------------+-------------------------------------------------
 {{{
 sage: R = LaurentPolynomialRing(ZZ, 'x')
 sage: T = LaurentPolynomialRing(QQ, 'x')
 sage: R.gen() + T.gen()
 ---------------------------------------------------------------------------
 TypeError                                 Traceback (most recent call
 last)
 <ipython-input-3-dac3b0b430b6> in <module>()
 ----> 1 R.gen() + T.gen()

 /home/darij/sage-5.13.beta0/local/lib/python2.7/site-
 packages/sage/structure/element.so in
 sage.structure.element.RingElement.__add__
 (sage/structure/element.c:13884)()

 /home/darij/sage-5.13.beta0/local/lib/python2.7/site-
 packages/sage/structure/coerce.so in
 sage.structure.coerce.CoercionModel_cache_maps.bin_op
 (sage/structure/coerce.c:8169)()

 TypeError: unsupported operand parent(s) for '+': 'Univariate Laurent
 Polynomial Ring in x over Integer Ring' and 'Univariate Laurent Polynomial
 Ring in x over Rational Field'
 }}}
 Compare with the more complicated:
 {{{
 sage: R = PolynomialRing(ZZ, 'x').fraction_field()
 sage: T = PolynomialRing(QQ, 'x').fraction_field()
 sage: R.gen() + T.gen()
 2*x
 }}}

 CCing Travis on this because it's a pain the ass for #15311.

--
Ticket URL: <http://trac.sagemath.org/ticket/15345>
Sage <http://www.sagemath.org>
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