Automatically generate default instances for all concrete subtypes of a type

New to Julia
3 
julia> using AbstractTrees
 
julia> struct TypeTree
           type::Type
       end
 
julia> function AbstractTrees.children(tree::TypeTree)
           Iterators.map(TypeTree, subtypes(tree.type))
       end
 
julia> abstract type A end
 
julia> struct P <: A end  # finitely-many subtypes
 
julia> struct Q <: A end  # finitely-many subtypes
 
julia> struct R{X} <: A end  # infinitely-many subtypes!
 
julia> leaf_subtypes = Iterators.map((x -> x.type), Leaves(TypeTree(A)));
 
julia> concrete_leaves = Iterators.filter(isconcretetype, leaf_subtypes);
 
julia> collect(Type, leaf_subtypes)
3-element Vector{Type}:
 P
 Q
 R
 
julia> collect(Type, concrete_leaves)
2-element Vector{Type}:
 P
 Q

Not sure what you mean.

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