test: improve coverage

Author: MilesCranmer

src/Expression.jl

@@ -100,8 +100,8 @@ end
     return Expression(tree, Metadata(d))
 end
 
-has_node_type(::Union{E,Type{E}}) where {N,E<:AbstractExpression{<:Any,N}} = true
-has_node_type(::Union{E,Type{E}}) where {E<:AbstractExpression} = false
+has_node_type(::Union{E,Type{E}}) where {N,E<:AbstractExpression{<:Any,N}} = true  # COV_EXCL_LINE
+has_node_type(::Union{E,Type{E}}) where {E<:AbstractExpression} = false  # COV_EXCL_LINE
 node_type(::Union{E,Type{E}}) where {N,E<:AbstractExpression{<:Any,N}} = N
 function max_degree(::Union{E,Type{E}}) where {E<:AbstractExpression}
     return has_node_type(E) ? max_degree(node_type(E)) : max_degree(Node)

src/base.jl

@@ -223,15 +223,16 @@ end
             leaf_equal(a, b)
         else
             (
-                branch_equal(a, b) && Base.Cartesian.@nif(
+                branch_equal(a, b) && Base.Cartesian.@nif(  # COV_EXCL_LINE
                     $D,
                     i -> deg == i,  # COV_EXCL_LINE
-                    i ->
+                    i -> begin  # COV_EXCL_LINE
                         let cs_a = get_children(a, Val(i)), cs_b = get_children(b, Val(i))  # COV_EXCL_LINE
                             Base.Cartesian.@nall(
                                 i, j -> inner_is_equal(cs_a[j], cs_b[j], id_maps)
                             )
                         end
+                    end
                 )
             )
         end

test/test_n_arity_nodes.jl

@@ -354,6 +354,23 @@ end
     tree_f64_d3_for_convert = tree_tmr
     converted_tree_f32_d3_explicit_D = convert(Node{Float32,3}, tree_f64_d3_for_convert)
     @test typeof(converted_tree_f32_d3_explicit_D) == Node{Float32,3}
+
+    # Test that converting between nodes with different degrees throws ArgumentError
+    tree_d2 = Node{Float64,2}(; val=1.0)  # Node with max degree 2
+    tree_d3 = Node{Float64,3}(; val=1.0)  # Node with max degree 3
+
+    @test_throws ArgumentError convert(Node{Float64,2}, tree_d3)
+    @test_throws ArgumentError convert(Node{Float64,3}, tree_d2)
+
+    # Verify the error message contains the expected text
+    try
+        convert(Node{Float64,2}, tree_d3)
+        @test false  # Should not reach here
+    catch e
+        @test e isa ArgumentError
+        @test occursin("Cannot convert", e.msg)
+        @test occursin("because they have different degrees", e.msg)
+    end
 end
 
 @testitem "LoopVectorizationExt with N-ary (degn_eval)" tags = [:narity] begin