fix: default_node_type should not prescribe degree

Author: MilesCranmer

src/Expression.jl

@@ -107,7 +107,7 @@ function max_degree(::Union{E,Type{E}}) where {E<:AbstractExpression}
     return has_node_type(E) ? max_degree(node_type(E)) : max_degree(Node)
 end
 @unstable default_node_type(_) = Node
-default_node_type(::Type{N}) where {T,N<:AbstractExpression{T}} = Node{T,max_degree(N)}
+default_node_type(::Type{N}) where {T,N<:AbstractExpression{T}} = Node{T}
 
 ########################################################
 # Abstract interface ###################################

src/Interfaces.jl

@@ -15,6 +15,7 @@ using ..NodeModule:
     with_type_parameters,
     with_max_degree,
     max_degree,
+    has_max_degree,
     unsafe_get_children,
     get_children,
     leaf_copy,
@@ -144,7 +145,8 @@ function _check_default_node(ex::AbstractExpression{T}) where {T}
     ET = typeof(ex)
     E = Base.typename(ET).wrapper
     return default_node_type(E) <: AbstractExpressionNode &&
-           default_node_type(ET) <: AbstractExpressionNode{T}
+           default_node_type(ET) <: AbstractExpressionNode{T} &&
+           !has_max_degree(default_node_type(ET))
 end
 function _check_constructorof(ex::AbstractExpression)
     return constructorof(typeof(ex)) isa Base.Callable

src/ParametricExpression.jl

@@ -119,12 +119,9 @@ end
 # Abstract expression node interface ##########################################
 ###############################################################################
 @unstable constructorof(::Type{<:ParametricExpression}) = ParametricExpression
-@unstable function default_node_type(::Type{<:ParametricExpression})
-    return with_default_max_degree(ParametricNode)
-end
-function default_node_type(::Type{N}) where {T,N<:ParametricExpression{T}}
-    return ParametricNode{T,max_degree(N)}
-end
+@unstable default_node_type(::Type{<:ParametricExpression}) = ParametricNode
+@unstable default_node_type(::Type{N}) where {T,N<:ParametricExpression{T}} =
+    ParametricNode{T}
 preserve_sharing(::Union{Type{<:ParametricNode},ParametricNode}) = false  # COV_EXCL_LINE
 function leaf_copy(t::ParametricNode{T}) where {T}
     if t.constant