Add many @req !is_trivial checks

Author: lgoettgens

src/MPoly.jl

@@ -1655,5 +1655,7 @@ true
 Like [`polynomial_ring(R::Ring, s::Vector{Symbol})`](@ref) but return only the
 multivariate polynomial ring.
 """
-polynomial_ring_only(R::T, s::Vector{Symbol}; internal_ordering::Symbol=:lex, cached::Bool=true) where T<:Ring =
-   mpoly_ring_type(T)(R, s, internal_ordering, cached)
+function polynomial_ring_only(R::T, s::Vector{Symbol}; internal_ordering::Symbol=:lex, cached::Bool=true) where T<:Ring
+   @req !is_trivial(R) "Zero rings are currently not supported as coefficient ring."
+   return mpoly_ring_type(T)(R, s, internal_ordering, cached)
+end

src/NCPoly.jl

@@ -756,8 +756,10 @@ end
 Like [`polynomial_ring(R::NCRing, s::Symbol)`](@ref) but return only the
 polynomial ring.
 """
-polynomial_ring_only(R::T, s::Symbol; cached::Bool=true) where T<:NCRing =
-   poly_ring_type(T)(R, s, cached)
+function polynomial_ring_only(R::T, s::Symbol; cached::Bool=true) where T<:NCRing
+   @req !is_trivial(R) "Zero rings are currently not supported as coefficient ring."
+   return poly_ring_type(T)(R, s, cached)
+end
 
 # Simplified constructor
 

src/Residue.jl

@@ -445,6 +445,7 @@ to the constructor with the same base ring $R$ and element $a$. A modulus
 of zero is not supported and throws an exception.
 """
 function residue_ring(R::Ring, a::RingElement; cached::Bool = true)
+   @req !is_trivial(R) "Zero rings are currently not supported as base ring."
    # Modulus of zero cannot be supported. E.g. A C library could not be expected to
    # do matrices over Z/0 using a Z/nZ type. The former is multiprecision, the latter not.
    iszero(a) && throw(DomainError(a, "Modulus must be nonzero"))
@@ -454,6 +455,7 @@ function residue_ring(R::Ring, a::RingElement; cached::Bool = true)
 end
 
 function residue_ring(R::PolyRing, a::RingElement; cached::Bool = true)
+   @req !is_trivial(R) "Zero rings are currently not supported as base ring."
    iszero(a) && throw(DomainError(a, "Modulus must be nonzero"))
    !is_unit(leading_coefficient(a)) && throw(DomainError(a, "Non-invertible leading coefficient"))
    T = elem_type(R)

src/generic/Misc/Localization.jl

@@ -113,6 +113,8 @@ base_ring(L::LocalizedEuclideanRing) = L.base_ring::base_ring_type(L)
 
 parent(a::LocalizedEuclideanRingElem) = a.parent
 
+characteristic(::LocalizedEuclideanRing) = characteristic(base_ring(L))
+
 ###############################################################################
 #
 #   Basic manipulation