Small union type failure

General Usage
6

I wonder if something along the lines of

function Base._promote_typejoin(::Type{S}, ::Type{T}) where {S, T}
    _cs(T, S) = (isconcretetype(T) || T ≡ Union{}) && Base.issingletontype(S)
    if _cs(T, S) || _cs(S, T)
        Union{T, S}
    else
        typejoin(S, T)
    end
end

would work, generalizing the existing cases to the union of a concrete and a singleton type.