guard against trait type piracy in a dependent package

Prevent dependent packages from commiting type piracy easily and
unintentionally when defining an AbstractTrees trait for all subtypes
of a type. Specifically: the bottom type, `Union{}`, is a subtype of
each type, so add a method for the bottom type for each AbstractTrees
trait. This method will take precedence over sane methods in dependent
packages, thus preventing spurious type piracy.

Author: nsajko

src/traits.jl

@@ -1,3 +1,4 @@
+const not_supported_exc = ArgumentError("not supported")
 
 """
     ParentLinks(::Type{T})
@@ -42,6 +43,7 @@ the tree structure and cannot be inferred through a single node.
 """
 struct ImplicitParents <: ParentLinks; end
 
+ParentLinks(::Type{Union{}}) = throw(not_supported_exc)
 ParentLinks(::Type) = ImplicitParents()
 ParentLinks(tree) = ParentLinks(typeof(tree))
 
@@ -84,6 +86,7 @@ from the tree structure.
 """
 struct ImplicitSiblings <: SiblingLinks; end
 
+SiblingLinks(::Type{Union{}}) = throw(not_supported_exc)
 SiblingLinks(::Type) = ImplicitSiblings()
 SiblingLinks(tree) = SiblingLinks(typeof(tree))
 
@@ -126,6 +129,7 @@ class of indexable trees consisting of arrays.
 """
 struct NonIndexedChildren <: ChildIndexing end
 
+ChildIndexing(::Type{Union{}}) = throw(not_supported_exc)
 ChildIndexing(::Type) = NonIndexedChildren()
 ChildIndexing(node) = ChildIndexing(typeof(node))
 
@@ -143,6 +147,7 @@ If the `childrentype` can be inferred from the type of the node alone, the type
 **OPTIONAL**: In most cases, [`childtype`](@ref) is used instead.  If `childtype` is not defined it will fall back
 to `eltype ∘ childrentype`.
 """
+childrentype(::Type{Union{}}) = throw(not_supported_exc)
 childrentype(::Type{T}) where {T} = Base._return_type(children, Tuple{T})
 childrentype(node) = typeof(children(node))
 
@@ -159,6 +164,7 @@ If `childtype` can be inferred from the type of the node alone, the type `::Type
 can be type-stable.  If `childrentype` is defined and can be known from the node type alone, this function will
 fall back to `eltype(childrentype(T))`.  If this gives a correct result it's not necessary to define `childtype`.
 """
+childtype(::Type{Union{}}) = throw(not_supported_exc)
 childtype(::Type{T}) where {T} = eltype(childrentype(T))
 childtype(node) = eltype(childrentype(node))
 
@@ -172,6 +178,7 @@ traversal is type stable.
 
 **OPTIONAL**: Type inference is used to attempt to
 """
+childstatetype(::Type{Union{}}) = throw(not_supported_exc)
 childstatetype(::Type{T}) where {T} = Iterators.approx_iter_type(childrentype(T))
 childstatetype(node) = childstatetype(typeof(node))
 
@@ -204,6 +211,7 @@ type.
 """
 struct NodeTypeUnknown <: NodeType end
 
+NodeType(::Type{Union{}}) = throw(not_supported_exc)
 NodeType(::Type) = NodeTypeUnknown()
 NodeType(node) = NodeType(typeof(node))
 
@@ -214,5 +222,6 @@ NodeType(node) = NodeType(typeof(node))
 Returns a type which must be a parent type of all nodes in the tree connected to `node`.  This can be used to,
 for example, specify the `eltype` of any `TreeIterator` on `node`.
 """
+nodetype(::Type{Union{}}) = throw(not_supported_exc)
 nodetype(::Type) = Any
 nodetype(node) = nodetype(typeof(node))

test/runtests.jl

@@ -1,6 +1,10 @@
 using AbstractTrees, Test
 using Aqua
 
+if Base.VERSION >= v"1.9"
+    # tests use `parentmodule(::Method)`, only supported on v1.9 and up
+    @testset "Traits" begin include("traits.jl") end
+end
 @testset "Builtins" begin include("builtins.jl") end
 @testset "Custom tree types" begin include("trees.jl") end
 if Base.VERSION >= v"1.6"

test/traits.jl

@@ -0,0 +1,46 @@
+module TestTraits
+
+using AbstractTrees
+using Test
+
+function we_own_the_module(m::Module)
+    ret = false
+    while m != Main
+        if m == AbstractTrees
+            ret = true
+            break
+        end
+        m = parentmodule(m)
+    end
+    ret
+end
+
+function we_own_the_method(m::Method)
+    we_own_the_module(parentmodule(m))
+end
+
+const traits = (
+    ParentLinks, SiblingLinks, ChildIndexing, childrentype, childtype, AbstractTrees.childstatetype, NodeType, nodetype,
+)
+
+struct T end
+
+for func ∈ traits
+    f = Symbol(func)
+    @eval begin
+        function AbstractTrees.$f(::Type{<:T})
+            # This method should not ever get called, it just serves to test dispatch/type piracy.
+            throw(ArgumentError("this is not the method you're looking for"))
+        end
+    end
+end
+
+@testset "Traits" begin
+    @testset "traits should not make dependents vulnerable to commiting type piracy" begin
+        @testset "func: $func" for func ∈ traits
+            @test all(we_own_the_method, methods(func, Tuple{Type{Union{}}}))
+        end
+    end
+end
+
+end