Inner type parameter depending on outer type parameter loses the fixed outer parameter information

[wsshin]

This problem was originally reported here in Julia discourse. The following is a simplified description of the problem through the discussion there.

Consider a parametrized type MyType with two type parameters N and S, where the range of the inner type parameter S depends on the outer type parameter N:

julia> VERSION
v"0.7.0-DEV.4465"

julia> struct MyType{N,S<:NTuple{N}}
           t::S
       end

Now, create instances of two different MyType types—one with a tuple of three Int64's and another with a tuple of three Float64's:

julia> mi = MyType{3,NTuple{3,Int64}}((1,2,3))
MyType{3,Tuple{Int64,Int64,Int64}}((1, 2, 3))

julia> mf = MyType{3,NTuple{3,Float64}}((1.0,2.0,3.0))
MyType{3,Tuple{Float64,Float64,Float64}}((1.0, 2.0, 3.0))

Note that both mi and mf have the outer type parameter N = 3, and therefore their inner type parameter S must be subtypes of NTuple{3}. However, the information of N in S is lost when an inhomogeneous array of mi and mf is constructed:

julia> vm = [mi, mf]; typeof(vm)
Array{MyType{3,S} where S<:(Tuple{Vararg{T,N}} where T) where N,1}

Notice that the information N = 3 is correctly passed to the element type MyType{3,S} here, but not in S<:(Tuple{Vararg{T,N}} where T) where N. As a result, the following generates an error:

julia> vm::Vector{<:MyType{3}}
ERROR: TypeError: typeassert: expected Array{#s1,1} where #s1<:(MyType{3,S} where S<:(Tuple{T,T,T} where T)), got Array{MyType{3,S} where S<:(Tuple{Vararg{T,N}} where T) where N,1}

even though both mi and mf are of type MyType{3}:

julia> mi::MyType{3}
MyType{3,Tuple{Int64,Int64,Int64}}((1, 2, 3))

julia> mf::MyType{3}
MyType{3,Tuple{Float64,Float64,Float64}}((1.0, 2.0, 3.0))

Is this an expected behavior?

[JeffBezanson]

This is the result given by typejoin, which is sometimes lazy and gives a wider type than it could. It should be improved in this case: instead of using the original typevar when corresponding parameters don't match, it should use the typevar obtained after substituting values for prior parameters.

[wsshin]

Thanks a lot for fixing this issue so quickly!