Use promotion in default isapprox methods

src/NCRings.jl

@@ -103,9 +103,9 @@ end
 # hand, we need isapprox methods to be able to conformance test series rings.
 # On the other hand this is essentially the only sensible thing to do in
 # positive characteristic so we might as well do it in a generic method.
-function Base.isapprox(x::NCRingElem, y::NCRingElem;
+function Base.isapprox(x::T, y::T;
                        atol::Real=0, rtol::Real=0,
-                       nans::Bool=false, norm::Function=abs)
+                       nans::Bool=false, norm::Function=abs) where {T <: NCRingElem}
   if is_exact_type(typeof(x)) && is_exact_type(typeof(y))
     @req is_zero(atol) "non-zero atol not supported"
     @req is_zero(rtol) "non-zero rtol not supported"
@@ -114,6 +114,21 @@ function Base.isapprox(x::NCRingElem, y::NCRingElem;
   throw(NotImplementedError(:isapprox, x, y))
 end
 
+function Base.isapprox(x::NCRingElem, y::NCRingElem; kwarg...)
+  fl, u, v = try_promote(x, y)
+  if fl
+    return isapprox(u, v; kwarg...)
+  end
+  throw(NotImplementedError(:isapprox, x, y))
+end
+
+Base.isapprox(x::NCRingElem, y::Union{Integer, Rational, AbstractFloat}; kwarg...)
+  = isapprox(x, parent(x)(y); kwarg...)
+
+Base.isapprox(x::Union{Integer, Rational, AbstractFloat}, y::NCRingElem; kwarg...)
+  = isapprox(parent(y)(x), y; kwarg...)
+
+
 function divexact_left(x::NCRingElem, y::NCRingElem; check::Bool = true)
    return divexact_left(promote(x, y)...)
 end