Dispatching on types with the same UnionAll (but you don't know the types beforehand). https://mcognetta.github.io/post/dispatch_unionall

Dispatch_UnionAll_code.jl

using SimpleTraits, Test

abstract type A{T} end
struct B{T} <: A{T} end
struct C{T} <: A{T} end
abstract type D{T} <: A{T} end
struct E{T} <: D{T} end
struct F{T} <: D{T} end

sameunionall(::Type{X}, ::Type{Y}) where {X<:A, Y<:A} = !isabstracttype(typejoin(X, Y))

@traitdef SameUnionAll{X, Y}
@traitimpl SameUnionAll{X, Y} <- sameunionall(X, Y)
@traitfn f(::X, ::Y) where {X<:A, Y<:A; SameUnionAll{X, Y}} = "yo"
@traitfn f(::X, ::Y) where {X<:A, Y<:A; !SameUnionAll{X, Y}} = "nah"

x = B{Int64}()
y = B{Float32}()
z = C{Int64}()
@test f(x, y) == "yo"
@test f(x, z) == "nah"
@test f(y, z) == "nah"

# added later by a user
struct G{T} <: D{T} end
@test f(G{Int16}(), x) == "nah"
@test f(G{Int16}(), G{BigFloat}()) == "yo"