LazyPowerSeriesRing.sum not working

[mantepse]

Steps To Reproduce

define:

R.<p, q> = QQ[]
L.<x> = LazyPowerSeriesRing(R)
@cached_function
def D_le(h):
    if not h:
        return 1
    P = L.undefined()
    P.define(x*(D_le(h-1) - 1 + p)*P + 1)
    return P

def D_eq(h):
    if not h:
        return 1
    return D_le(h) - D_le(h-1)

D = L.sum(lambda h: q^h*D_eq(h), 0)

Expected Behavior

sage: D
1 + p*q*x + ((p^2*q+p*q^2)*x^2) + ((p^3*q+3*p^2*q^2+p*q^3)*x^3) + ((p^4*q+6*p^3*q^2+5*p^2*q^3+p*q^4+p^2*q^2)*x^4) + ((p^5*q+10*p^4*q^2+15*p^3*q^3+7*p^2*q^4+p*q^5+5*p^3*q^2+3*p^2*q^3)*x^5) + ((p^6*q+15*p^5*q^2+35*p^4*q^3+28*p^3*q^4+9*p^2*q^5+p*q^6+15*p^4*q^2+21*p^3*q^3+5*p^2*q^4+p^3*q^2+p^2*q^3)*x^6) + O(x^7)

Actual Behavior

we actually have

sage: D
<repr(<sage.rings.lazy_series_ring.LazyPowerSeriesRing_with_category.element_class at 0x7f670bf99cc0>) failed: AttributeError: 'sage.rings.polynomial.multi_polynomial_libsingular.MPolynomial_libsingular' object has no attribute '_coeff_stream'. Did you mean: '_coeff_repr'?>
sage: D
p*q*x + ((p^2*q+p*q^2)*x^2) + ((p^3*q+3*p^2*q^2+p*q^3)*x^3) + ((p^4*q+6*p^3*q^2+5*p^2*q^3+p*q^4+p^2*q^2)*x^4) + ((p^5*q+10*p^4*q^2+15*p^3*q^3+7*p^2*q^4+p*q^5+5*p^3*q^2+3*p^2*q^3)*x^5) + ((p^6*q+15*p^5*q^2+35*p^4*q^3+28*p^3*q^4+9*p^2*q^5+p*q^6+15*p^4*q^2+21*p^3*q^3+5*p^2*q^4+p^3*q^2+p^2*q^3)*x^6) + O(x^7)

• the first invocation of D fails
• the second is not quite correct, the constant term is missing.

Additional Information

A workaround is to define

D = L.sum(lambda h: L(q^h)*D_eq(h), 0)

Environment

- **Sage Version**: 10.3.beta8

Checklist

• [X] I have searched the existing issues for a bug report that matches the one I want to file, without success.
• [X] I have read the documentation and troubleshoot guide

[tscrim]

Strictly speaking, this isn't a bug but invalid input. The base case is not returning an element in L, and the input to sum is expecting an element of L from the function. So if you change the return 1 -> return L.one(), it works.

That being said, it would be desirable to automatically coerce (convert?) the inputs into L, but that would require the stream to know something about the parent.