fix: node conversion changing degree

Author: MilesCranmer

src/Node.jl

@@ -218,13 +218,20 @@ end
 Base.eltype(::Type{<:AbstractExpressionNode{T}}) where {T} = T
 Base.eltype(::AbstractExpressionNode{T}) where {T} = T
 
-max_degree(::Type{<:AbstractNode}) = 2  # Default
+const DEFAULT_MAX_DEGREE = 2
+max_degree(::Type{<:AbstractNode}) = DEFAULT_MAX_DEGREE
 max_degree(::Type{<:AbstractNode{D}}) where {D} = D
 max_degree(node::AbstractNode) = max_degree(typeof(node))
 
-@unstable constructorof(::Type{N}) where {N<:Node} = Node{T,max_degree(N)} where {T}
-@unstable constructorof(::Type{N}) where {N<:GraphNode} =
-    GraphNode{T,max_degree(N)} where {T}
+has_max_degree(::Type{<:AbstractNode}) = false
+has_max_degree(::Type{<:AbstractNode{D}}) where {D} = true
+
+@unstable function constructorof(::Type{N}) where {N<:Node}
+    return Node{T,max_degree(N)} where {T}
+end
+@unstable function constructorof(::Type{N}) where {N<:GraphNode}
+    return GraphNode{T,max_degree(N)} where {T}
+end
 
 function with_type_parameters(::Type{N}, ::Type{T}) where {N<:Node,T}
     return Node{T,max_degree(N)}
@@ -233,6 +240,9 @@ function with_type_parameters(::Type{N}, ::Type{T}) where {N<:GraphNode,T}
     return GraphNode{T,max_degree(N)}
 end
 
+with_max_degree(::Type{N}, ::Val{D}) where {T,N<:Node{T},D} = Node{T,D}
+with_max_degree(::Type{N}, ::Val{D}) where {T,N<:GraphNode{T},D} = GraphNode{T,D}
+
 function default_allocator(::Type{N}, ::Type{T}) where {N<:AbstractExpressionNode,T}
     return with_type_parameters(N, T)()
 end

src/base.jl

@@ -520,10 +520,11 @@ using `convert(T1, tree.val)` at constant nodes.
 """
 function convert(
     ::Type{N1}, tree::N2
-) where {T1,T2,N1<:AbstractExpressionNode{T1},N2<:AbstractExpressionNode{T2}}
+) where {T1,T2,D1,D2,N1<:AbstractExpressionNode{T1,D1},N2<:AbstractExpressionNode{T2,D2}}
     if N1 === N2
         return tree
     end
+    @assert max_degree(N1) == max_degree(N2)
     return tree_mapreduce(
         Base.Fix1(leaf_convert, N1),
         identity,
@@ -533,6 +534,11 @@ function convert(
     )
     # TODO: Need to allow user to overload this!
 end
+function convert(
+    ::Type{N1}, tree::N2
+) where {T1,T2,D2,N1<:AbstractExpressionNode{T1},N2<:AbstractExpressionNode{T2,D2}}
+    return convert(with_max_degree(N1, Val(D2)), tree)
+end
 function convert(
     ::Type{N1}, tree::N2
 ) where {T2,N1<:AbstractExpressionNode,N2<:AbstractExpressionNode{T2}}