Coercion works incorrectly for power series over power series

[haraldschilly]

This was reported via the "Report a Problem" link:

When I try the following:

S1 = LazyPowerSeriesRing(ZZ)
S2 = LazyPowerSeriesRing(S1)

t = S1.term(1, 1) # same results with t = S1.gen()
tt = S2(t)
tt.coefficient(0)

this yields 0. Trying

tt.aorder

I get 1.

The series tt is the series with constant term equal to t, so its order should be zero and its 0th coefficient should be t.

CC: axel.bacher@labri.fr @mwhansen @mantepse

Component: combinatorics

Keywords: LazyPowerSeries

Stopgaps: todo

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/10086

[mantepse]

Changed keywords from none to LazyPowerSeries

[mwhansen]

comment:4

A fix is in #15673