Improve conversion

Author: blegat

src/conversion.jl

@@ -1,8 +1,13 @@
 export variable
 
-convertconstant(::Type{P}, α) where P<:APL = P(α)
+function convertconstant end
 Base.convert(::Type{P}, α) where P<:APL = convertconstant(P, α)
-Base.convert(::Type{P}, p::APL) where P<:AbstractPolynomial = P(polynomial(p))
+function Base.convert(::Type{P}, p::P) where P<:AbstractPolynomial
+    return p
+end
+function Base.convert(::Type{P}, p::APL) where {T, P<:AbstractPolynomial{T}}
+    return convert(P, polynomial(p, T))
+end
 
 Base.convert(::Type{Any}, p::APL) = p
 # Conversion polynomial -> scalar

test/algebra.jl

@@ -40,6 +40,14 @@
     @test p == x^3 + 2x^2*y + x + 2y
     @test p isa AbstractPolynomial{Float64}
 
+    p = @inferred CustomPoly(x + 2y) + 1
+    @test p == x + 2y + 1
+    @test p isa AbstractPolynomial{Int}
+
+    p = @inferred CustomPoly(x + 2y) + 1.0
+    @test p == x + 2y + 1
+    @test p isa AbstractPolynomial{Float64}
+
     @testset "Inference" begin
         @inferred x^2
         @inferred x^2-2x

test/utils.jl

@@ -3,12 +3,19 @@ struct CustomPoly{T, P<:AbstractPolynomial{T}} <: AbstractPolynomialLike{T}
 end
 CustomPoly(p::AbstractPolynomial{T}) where T = CustomPoly{T, typeof(p)}(p)
 MultivariatePolynomials.polynomial(p::CustomPoly) = p.p
+MultivariatePolynomials.polynomial(p::CustomPoly, T::Type) = polynomial(p.p, T)
+MultivariatePolynomials.variables(p::CustomPoly) = variables(p.p)
+MultivariatePolynomials.monomialtype(::Type{<:CustomPoly{T, P}}) where {T, P} = monomialtype(P)
+MultivariatePolynomials.constantmonomial(p::CustomPoly) = constantmonomial(p.p)
 
 struct CustomTerms{T, P<:AbstractPolynomial{T}} <: AbstractPolynomialLike{T}
     p::P
 end
 CustomTerms(p::AbstractPolynomial{T}) where T = CustomTerms{T, typeof(p)}(p)
 MultivariatePolynomials.terms(p::CustomTerms) = terms(p.p)
+MultivariatePolynomials.variables(p::CustomTerms) = variables(p.p)
+MultivariatePolynomials.monomialtype(::Type{<:CustomTerms{T, P}}) where {T, P} = monomialtype(P)
+MultivariatePolynomials.constantmonomial(p::CustomPoly) = constantmonomial(p.p)
 
 function _typetests(x, ::Type{T}) where T
     @test (@inferred coefficienttype(x)) == Int