fix: some issues with D-degree ParametricNode

Author: MilesCranmer

src/ParametricExpression.jl

@@ -8,7 +8,7 @@ using ..NodeModule: AbstractExpressionNode, Node, tree_mapreduce
 using ..ExpressionModule: AbstractExpression, Metadata
 using ..ChainRulesModule: NodeTangent
 
-import ..NodeModule: constructorof, preserve_sharing, leaf_copy, leaf_hash, leaf_equal
+import ..NodeModule: constructorof, max_degree, preserve_sharing, leaf_copy, leaf_hash, leaf_equal
 import ..NodeUtilsModule:
     count_constant_nodes,
     index_constant_nodes,
@@ -96,10 +96,10 @@ end
 ###############################################################################
 # Abstract expression node interface ##########################################
 ###############################################################################
-@unstable constructorof(::Type{<:ParametricNode}) = ParametricNode
+@unstable constructorof(::Type{N}) where {N<:ParametricNode} = ParametricNode{T,max_degree(N)} where {T}
 @unstable constructorof(::Type{<:ParametricExpression}) = ParametricExpression
-@unstable default_node_type(::Type{<:ParametricExpression}) = ParametricNode
-default_node_type(::Type{<:ParametricExpression{T}}) where {T} = ParametricNode{T}
+@unstable default_node_type(::Type{<:ParametricExpression}) = ParametricNode{T,2} where {T}
+default_node_type(::Type{<:ParametricExpression{T}}) where {T} = ParametricNode{T,2}
 preserve_sharing(::Union{Type{<:ParametricNode},ParametricNode}) = false # TODO: Change this?
 function leaf_copy(t::ParametricNode{T}) where {T}
     out = if t.constant