fix: constructor from explicit eltype

Author: MilesCranmer

src/Node.jl

@@ -299,16 +299,18 @@ end
 end
 
 @inline function node_factory_type(::Type{N}, ::Type{T1}, ::Type{T2}) where {N,T1,T2}
-    if T1 === Undefined && N isa UnionAll
+    if T1 === Undefined && !defines_eltype(N)
         T2
     elseif T1 === Undefined
         eltype(N)
-    elseif N isa UnionAll
+    elseif !defines_eltype(N)
         T1
     else
         eltype(N)
     end
 end
+defines_eltype(::Type{<:AbstractExpressionNode}) = false
+defines_eltype(::Type{<:AbstractExpressionNode{T}}) where {T} = true
 #! format: on
 
 function (::Type{N})(

test/test_base_2.jl

@@ -12,6 +12,7 @@
     using DynamicExpressions, Random
 
     x = Node{Float64}(; feature=1)
+    @test x isa Node{Float64}
 
     # We can also create values, using `val`:
     const_1 = Node{Float64}(; val=1.0)

test/test_parametric_expression.jl

@@ -349,7 +349,7 @@ end
     @test val isa Float64
     @test grad isa NamedTuple
     @test grad.tree isa DynamicExpressions.ChainRulesModule.NodeTangent{
-        Float64,ParametricNode{Float64},Vector{Float64}
+        Float64,<:ParametricNode{Float64},Vector{Float64}
     }
     @test grad.metadata._data.parameters isa Matrix{Float64}