collect doesn't preserve shape on Broadcased objects

[oxinabox]

consider a 2D generator

julia> g = (1 for x in 1:2, y in 2:3)
Base.Generator{Base.Iterators.ProductIterator{Tuple{UnitRange{Int64}, UnitRange{Int64}}}, var"#47#48"}(var"#47#48"(), Base.Iterators.ProductIterator{Tuple{UnitRange{Int64}, UnitRange{Int64}}}((1:2, 2:3)))

julia> size(g)
(2, 2)

julia> collect(g)
2×2 Matrix{Int64}:
 1  1
 1  1

All is well.

But for a Broadcasted object the shape is not preserved and collect gives back something flat.

julia> h = Base.broadcasted(sqrt, [1 2; 3 4])
Base.Broadcast.Broadcasted(sqrt, ([1 2; 3 4],))

julia> size(h)
(2, 2)

julia> collect(h)
4-element Vector{Float64}:
 1.0
 1.7320508075688772
 1.4142135623730951
 2.0

This can be fixed by materializing first

julia> collect(Base.materialize(h))
2×2 Matrix{Float64}:
 1.0      1.41421
 1.73205  2.0

But why does it not work in the first place.
I suspect it is because the IteratorSize trait isn't set to HasShape.
But why isn't it?

julia> Base.IteratorSize(g)
Base.HasShape{2}()

julia> Base.IteratorSize(h)
Base.HasLength()

It seems to just be hitting the default fallback defintion of IteratorSize
rather than:

julia/base/broadcast.jl

Line 264 in 591f066

Base.IteratorSize(::Type{<:Broadcasted{<:Any,<:NTuple{N,Base.OneTo}}}) where {N} = Base.HasShape{N}()

We do have:

julia> typeof(h)
Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{2}, Nothing, typeof(sqrt), Tuple{Matrix{Int64}}}

And that Base.Broadcast.DefaultArrayStyle{2} tells us what we need to know to define the HasShape

It seems like we could define

IteratorSize(::Type{<:Broadcasted{<:AbstractArrayStyle{N}}}) where {N} = HasShape{N}()

and indeed that does seem to work

julia> collect(h)
2×2 Matrix{Float64}:
 1.0      1.41421
 1.73205  2.0

julia> Base.IteratorSize(::Type{<:Base.Broadcast.Broadcasted{<:Base.Broadcast.AbstractArrayStyle{N}}}) where {N} = Base.HasShape{N}()

julia> collect(h)
2×2 Matrix{Float64}:
 1.0      1.41421
 1.73205  2.0

[tkf]

You forgot instantiate

julia> h = Base.broadcasted(sqrt, [1 2; 3 4])
Base.Broadcast.Broadcasted(sqrt, ([1 2; 3 4],))

julia> h2 = Broadcast.instantiate(h)
Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{2}}(sqrt, ([1 2; 3 4],))

julia> collect(h2)
2×2 Matrix{Float64}:
 1.0      1.41421
 1.73205  2.0

[oxinabox]

How so?

Without instantiate it still defines size and is iterable.
We should honor it's size

[vtjnash]

Agreed. In particular, instantiate is not supposed to have visible side-effects in this way, but merely pre-computes useful information. Can we entirely replace that Base.IteratorSize definition? It seems like it may otherwise be ambiguous (in some cases) with the proposed fix here?

[oxinabox]

I am not sure if we can replace it, because it might be being hit by something that is otherwise overloading the broadcast style in a package.
So we may need to include the disambiguating case

[oxinabox]

Effectively closed by #44061
though some issues with that lead to #45477 being opened .