Fix inference issue for is_commutative

src/variable.jl

@@ -56,7 +56,13 @@ is_commutative(p::_APL) = is_commutative(typeof(p))
 is_commutative(v::AbstractVector) = is_commutative(eltype(v))
 is_commutative(::Type{V}) where {V<:AbstractVector} = is_commutative(eltype(V))
 is_commutative(v::Tuple) = all(is_commutative, v)
-is_commutative(::Type{T}) where {T<:Tuple} = all(is_commutative, fieldtypes(T))
+# Recursive dispatch allows the compiler to constant-fold for concrete Tuple types,
+# enabling type-stable branches on `is_commutative(V)` in downstream packages.
+is_commutative(::Type{Tuple{}}) = true
+function is_commutative(::Type{T}) where {T<:Tuple}
+    return is_commutative(Base.tuple_type_head(T)) &&
+           is_commutative(Base.tuple_type_tail(T))
+end
 
 """
     name(v::AbstractVariable)::AbstractString

test/Project.toml

@@ -11,6 +11,6 @@ Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 TypedPolynomials = "afbbf031-7a57-5f58-a1b9-b774a0fad08d"
 
 [compat]
-DynamicPolynomials = "0.6"
+DynamicPolynomials = "0.6.5"
 StarAlgebras = "0.3"
-TypedPolynomials = "0.4.2"
+TypedPolynomials = "0.4.4"

test/commutative/variable.jl

@@ -13,6 +13,10 @@ import MultivariatePolynomials:
         _is_comm_test(x^2)
         _is_comm_test(2x^2)
         _is_comm_test(sum(y))
+        # Test that `is_commutative` is constant-folded for variable types,
+        # so that branches on it are eliminated at compile time.
+        _comm_branch(::Type{V}) where {V} = MP.is_commutative(V) ? Int : String
+        @test @inferred(_comm_branch(typeof(variables(x * y[1])))) === Int
     end
     @testset "polyvar macro index set" begin
         Mod.@polyvar x y z