feat: prepare for fixes for polynomial rings over zero rings (#4350)

src/Rings/MPolyMap/flattenings.jl

@@ -169,7 +169,12 @@
     kk = coefficient_ring(R)::Field
 
     G = grading_group(S)
-    w = degree.(gens(S))
+    if !is_trivial(S)
+      w = degree.(gens(S))
+    else
+      # if S is trivial, the weights don't matter
+      w = [zero(G) for i in 1:ngens(S)]
+    end
     new_w = vcat(w, [zero(G) for i in 1:ngens(R)])
 
     # Before building S_flat, we have to create a polynomial 

src/Rings/mpoly-graded.jl

@@ -743,17 +743,17 @@ AbstractAlgebra.exponent(a::MPolyDecRingElem, i::Int, j::Int, ::Type{T}) where T
 
 function has_weighted_ordering(R::MPolyDecRing)
   grading_to_ordering = false
-  w_ord = degrevlex(gens(R)) # dummy, not used
+  w_ord = degrevlex(R) # dummy, not used
   # This is not meant to be exhaustive, there a probably more gradings which one
   # can meaningfully translate into a monomial ordering
   # However, we want to stick to global orderings.
   if is_z_graded(R)
     w = Int[ R.d[i].coeff[1] for i = 1:ngens(R) ]
     if all(isone, w)
-      w_ord = degrevlex(gens(R))
+      w_ord = degrevlex(R)
       grading_to_ordering = true
     elseif all(>(0), w)
-      w_ord = wdegrevlex(gens(R), w)
+      w_ord = wdegrevlex(R, w)
       grading_to_ordering = true
     end
   end