Fix bug in zero relative series to power zero. (#938)

* Fix bug in zero relative series to power zero.

Author: wbhart

src/RelSeries.jl

@@ -668,15 +668,15 @@ Return $a^b$. We require $b \geq 0$.
 function ^(a::RelSeriesElem{T}, b::Int) where T <: RingElement
    b < 0 && throw(DomainError(b, "exponent must be >= 0"))
    # special case powers of x for constructing power series efficiently
-   if pol_length(a) == 0
+   if b == 0
+      # in fact, the result would be exact 1 if we had exact series
+      z = one(parent(a))
+      return z
+   elseif pol_length(a) == 0
       z = parent(a)()
       z = set_precision!(z, b*valuation(a))
       z = set_valuation!(z, b*valuation(a))
       return z
-   elseif b == 0
-      # in fact, the result would be exact 1 if we had exact series
-      z = one(parent(a))
-      return z
    elseif isgen(a)
       z = parent(a)()
       fit!(z, 1)

test/generic/RelSeries-test.jl

@@ -616,6 +616,12 @@ end
       end
    end
 
+   # regression test (see #931)
+   Zn = ResidueRing(ZZ, 2)
+   R, x = PowerSeriesRing(Zn, 10, "x")
+   f = O(x^2)
+   @test isequal(f^0, 1 + O(x^10))
+
    f = rand(R, 0:12, 0:rand(1:25))
    @test_throws DomainError f^-1
    @test_throws DomainError f^-rand(2:100)