Fix gcd with zero (#177)

Author: blegat

src/gcd.jl

@@ -107,8 +107,8 @@ function Base.gcd(p1::APL{T}, p2::APL{S}, algo::AbstractUnivariateGCDAlgorithm=G
     if isapproxzero(p1)
         return convert(MA.promote_operation(gcd, typeof(p1), typeof(p2)), p2)
     end
-    if isapproxzero(p1)
-        return convert(MA.promote_operation(gcd, typeof(p1), typeof(p2)), p2)
+    if isapproxzero(p2)
+        return convert(MA.promote_operation(gcd, typeof(p1), typeof(p2)), p1)
     end
     shift1, defl1 = deflation(p1)
     shift2, defl2 = deflation(p2)

test/division.jl

@@ -134,6 +134,9 @@ end
 function multivariate_gcd_test(::Type{T}, algo=GeneralizedEuclideanAlgorithm()) where {T}
     Mod.@polyvar x y z
     o = one(T)
+    zr = zero(T)
+    sym_test(o * x, zr * x, o * x, algo)
+    sym_test(o * x + o, zr * x, o * x + o, algo)
     # Inspired from https://github.com/JuliaAlgebra/MultivariatePolynomials.jl/issues/160
     f1 = o * x * y + o * x
     f2 = o * y^2