Include a method for derivative of constant ImmutableDensePolynomial (N=1). The method returns the zero ImmutableDensePolynomial. This is a possible fix of a bug, since previously derivative of an ImmutablePolynomial returned an unusable polynomial of type ImmutablePolynomial{Union{},:x,0}`.

Author: iojea

src/polynomials/standard-basis/immutable-polynomial.jl

@@ -142,6 +142,9 @@ end
 
 # special cases are much more performant
 derivative(p::ImmutableDensePolynomial{B,T,X,0}) where {B<:StandardBasis,T,X} = p
+function derivative(p::ImmutableDensePolynomial{B,T,X,1}) where {B<:StandardBasis,T,X}
+    ImmutableDensePolynomial{B,T,X,1}(zero(T))
+end
 function derivative(p::ImmutableDensePolynomial{B,T,X,N}) where {B<:StandardBasis,T,X,N}
     N == 0 && return p
     hasnan(p) && return ImmutableDensePolynomial{B,T,X,1}(zero(T)/zero(T)) # NaN{T}