Hi there! I have defined a custom type which includes both a vector of matrices, and a vector of matrix pairs, and am trying to achieve type stability. Here is a simplified version of my code:

```
using LinearAlgebra, InteractiveUtils
struct MatrixPair{U<:AbstractMatrix{<:Real}}
A::U
B::U
end
struct Matrices{T<:Real, U<:AbstractMatrix{T}, V<:AbstractVector{U}, W<:AbstractVector{MatrixPair{U}}}
scalar::T
matrices::V
matrix_pairs::W
end
function sum(c::T, m::Matrices{T}) where T
m.matrices[1] + m.matrix_pairs[1].A + m.matrix_pairs[1].B .+ (c + m.scalar)
end
M(m) = fill(Float64(m), 2, 2)
mp1 = MatrixPair(M(3), M(4))
mp2 = MatrixPair(M(5), M(6))
mat = Matrices(1.5, [M(1)], [mp1, mp2])
@code_warntype sum(3.14, mat)
display(sum(3.14, mat))
```

This works well in Julia 0.7, but in 0.6 (which I have to use for this project) I get:

```
MethodError: no method matching Matrices(::Float64, ::Array{Array{Float64,2},1}, ::Array{MatrixPair{Array{Float64,2}},1})
```

From what I can tell, this is due to no field in `Matrices`

having type `U`

, and a workaround is to create such a field in the `Matrices`

struct: `dummy::U`

, but this seems silly. Is there a better way? Can the type definition above be simplified/improved while keeping the type stability? Thanks!