Don't allow grade on graded polynomial ring

src/Rings/mpoly-graded.jl

@@ -8,6 +8,7 @@
   multi_hilbert_series_parent::Generic.LaurentMPolyWrapRing{ZZRingElem, ZZMPolyRing}
 
   function MPolyDecRing(R::S, d::Vector{FinGenAbGroupElem}) where {S}
+    @req !(R isa MPolyDecRing) "cannot graded polynomial ring which is already decorated"
     @assert length(d) == ngens(R)
     r = new{elem_type(base_ring(R)), S}()
     r.R = R
@@ -16,6 +17,7 @@
     return r
   end
   function MPolyDecRing(R::S, d::Vector{FinGenAbGroupElem}, lt) where {S}
+    @req !(R isa MPolyDecRing) "cannot filter polynomial ring which is already decorated"
     @assert length(d) == ngens(R)
     r = new{elem_type(base_ring(R)), S}()
     r.R = R
@@ -135,7 +137,7 @@ of `R`, and return the new ring, together with the vector of variables.
 As above, where the grading is the standard $\mathbb Z$-grading on `R`.
 
 # Examples
-```jldoctest
+```jldoctest grade-ex
 julia> R, (x, y, z) = polynomial_ring(QQ, [:x, :y, :z])
 (Multivariate polynomial ring in 3 variables over QQ, QQMPolyRingElem[x, y, z])
 
@@ -156,7 +158,12 @@ Multivariate polynomial ring in 3 variables over QQ graded by
   x -> [1]
   y -> [1]
   z -> [1]
+```
 
+Grading an already graded polynomial ring is not supported.
+```jldoctest grade-ex
+julia> grade(S)
+ERROR: ArgumentError: cannot graded polynomial ring which is already decorated
 ```
 """
 function grade(R::MPolyRing, W::AbstractVector{<:IntegerUnion})

test/Serialization/PolynomialsSeries.jl

@@ -54,12 +54,6 @@ cases = [
         @test loaded == m1 * m2
         @test grading_group(parent(loaded)) == grading_group(M)
       end
-
-      GM, _ = grade(M, A)
-      test_save_load_roundtrip(path, GM) do loaded
-        @test loaded == GM
-        @test forget_grading(loaded) == forget_grading(GM)
-      end
     end
 
     for case in cases

test/Serialization/upgrades/setup_tests.jl

@@ -6,7 +6,7 @@ if !isdefined(Main, :serialization_upgrade_test_path) ||
   !isdir(Main.serialization_upgrade_test_path) ||
   !isfile(joinpath(Main.serialization_upgrade_test_path, "LICENSE.md"))
 
-  serialization_upgrade_test_path = let commit_hash = "a28d9e4dcc89de46377061e2f42ee09739735b97"
+  serialization_upgrade_test_path = let commit_hash = "ec819b9ddc0fc2bd0b842cfb1ea2341bd34beff4"
     tarball = Downloads.download("https://github.com/oscar-system/serialization-upgrade-tests/archive/$(commit_hash).tar.gz")
 
     destpath = open(CodecZlib.GzipDecompressorStream, tarball) do io