setindex! with AbstractVector should work with iterable objects

[ronisbr]

Hi!

I have a class Quaternion in ReferenceFrameRotations.jl that was <:AbstractVector (4 elements). Hence, the following code used to work perfectly:

julia> v[4:7] = q

However, I had many problems in Zygote because my class was an array and the multiplication of two quaternions (2 4x1 vectors) provides another quaternion (4x1 vector). This behavior was breaking a lot of things.

Thus, I remove the super type <:AbstractVector and defined all the things necessary to make Quaternion and iterable object. In this case, things like:

julia> v[4:7] .= q

works. But

julia> v[4:7] = q

does not.

I am wondering if this is a bug or not. Notice that if RHS is iterable, maybe Julia has all the information to see if it can be assigned to LHS. My current workaround is to define:

@inline function setindex!(v::Vector{T}, q::Quaternion, I::UnitRange) where T
    # We can use all the funcion in static arrays.
    return setindex!(v, q[:], I)
end

(q[:] converts a Quaternion into a Vector).

However, I am not sure if I am doing type piracy here.

[adienes]

I think this is intentionally not supported in the general case

julia> v = collect(1:10);

julia> v[4:6] = Set([1,2,3])
ERROR: ArgumentError: indexed assignment with a single value to possibly many locations is not supported; perhaps use broadcasting `.=` instead?

the more idiomatic way to do this might be to use splice!(v, 4:6, Set([1,2,3])) (although unfortunately this does allocate the output)