Support broadcast assignment to Ref objects

[jmert]

This is a pretty straight-forward extension of adding CartesianIndex() access in #32653 — I was motivated by trying to develop code which exploits broadcasting to operate on anything from scalars to multidimensional arrays using CartesianIndices indexing. This doesn't work for a Ref-wrapped scalar without these definitions, though.

[mbauman]

If the analogy is that Refs are like 0-dimensional arrays, then shouldn't this just return a Ref right back at you?

julia> A = fill(1)
0-dimensional Array{Int64,0}:
1

julia> A[CartesianIndices(())]
0-dimensional Array{Int64,0}:
1

Or perhaps more to the point, if we support this, I think we should support all AbstractArray{CartesianIndex{0},0}s. But I'd be very hesitant to do either.

[jmert]

Yes, it seems you're right — dropping the Ref wrapper isn't what I want. With getindex unwrapping, I hit a new error in broadcasting, and it's the same as if the code assumes the left hand side is a Ref:

julia> g1!(x) = x[] .= 1.0
g1! (generic function with 1 method)

julia> g1!(Ref(1.0))
ERROR: MethodError: no method matching copyto!(::Float64, ::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{0},Tuple{},typeof(identity),Tuple{Float64}})
...

Maybe I should poke at the broadcasting behavior instead?

[jmert]

It looks like if I instead define:

@eval Base begin
    maybeview(A::Ref, ::CartesianIndex{0}) = A
    maybeview(A::Ref, ::CartesianIndices{0,Tuple{}}) = A
end
@eval Base.Broadcast begin
    @inline function copyto!(dest::Ref, bc::Broadcasted{<:AbstractArrayStyle{0}})
        dest[] = materialize(bc)
    end
end

I can get the behavior I wanted with something like:

julia> g1!(x) = (x .= 1.0; x)
g1! (generic function with 1 method)

julia> g1!(Ref(0.0))
Base.RefValue{Float64}(1.0)

Is this a better idea? I suppose the alternative is to replicate a Ref-like struct and add similar definitions to it instead.