Using parametrised type in a Union breaks subtype check

[andreyz4k]

I want to allow my function to take two arguments. One is the usual of variable type T and the second one may be either of the same type T or it can be my custom container type Cont{T}. Unfortunately, it looks like in this case type checker decides to turn off the diagonal type checks. So here are refined examples:

julia> Tuple{Int64,Float64} <: Tuple{T,T} where T
false      # normal
julia> Tuple{Int64,Float64} <: Tuple{T,Union{Array{T}, T}} where T
true       # breaking
julia> Tuple{Int64,Array{Float64}} <: Tuple{T,Union{Array{T}, T}} where T
true       # breaking
julia> Tuple{Int64,Array{Float64}} <: Tuple{T,Union{Array{T}, T}} where T <: Number
false      # adding type constraint here helps for some reason
julia> Tuple{Int64,Float64} <: Tuple{T,Union{Array{T}, T}} where T<:Number
true       # but here it doesn't

Reproduces both on 1.5.1 and on the master branch versions.

[JeffBezanson]

This was an intentional change (#34272) needed to fix bugs. I agree it's inconvenient though. It might be possible to make this case work without totally reverting that change.

[julbinb]

Intuitively, Tuple{T,Union{Cont{T}, T}} where T should be the same as Union{Tuple{T,T} where T, Tuple{Cont{T}, T} where T}. One could probably get away by defining two methods, f(::T, ::T) where T and f(::T, ::Cont{T}) where T, and calling the former from the latter to avoid code duplication. Of course, the latter method allows abstract T in the container whereas the former does not.