known subtyping issue

[vtjnash]

julia> let a = Tuple{T3,Int64,Tuple{T3}} where T3,
           b = Tuple{S3,S3,S4} where S4 where S3,
           x = Tuple{Int64,Int64,Tuple{Int64}}
           typeintersect(a, b) <: a
       end
false # subtype is wrong about (Tuple{S3,S3,S4} where S4<:Tuple{Int64} where S3<:Int64) -- possibly a normalization issue?

julia> let a = Tuple{T1,Val{T2},T2} where T2 where T1,
           b = Tuple{Float64,S1,S2} where S2 where S1,
           x = Tuple{Float64,Val{Int64},Int64}
           typeintersect(a, b) <: a
       end
false # suboptimal, but valid (type-intersection was over-approximated)

from #19998

[nsajko]

The first example now returns true.

[vtjnash]

The second example also has the wrong value for x, it should be x = Tuple{Float64,Val{T2},T2} where T2 <: Int. The first example, similarly, may be only correct now because we disallow Union{} being an element. Otherwise this precise intersection gets harder to express, as it should be Tuple{S3, Int64, S4} where {S3<:Int64, S4<:Tuple{S3}}, which excludes Tuple{Union{}, Int64, Tuple{Int64}} from the set of valid intersections in the return value.

In v1.10, before we fixed that, this example violated transitivity:

julia> x <: a
true

julia> x <: b
true

julia> y <: a
true

julia> y <: b
false

julia> y <: x
true

julia> x <: y
true

julia> y <: x
true