Broadcasting flatten with type arguments causes type-instability

[simonbyrne]

If Base.Broadcast.flatten is called on a Broadcasted object with a DataType f, the resulting Broadcasted object is no longer type-stable, triggering allocations.

This can be triggered when using || or && in expressions, a simple reproducible example of this:

julia> x = rand(5); y = rand(5);

julia> f!(y, x) = @. y = ifelse(false || x < Float32(1), x, y)
f! (generic function with 1 method)

julia> f!(y, x);

julia> @allocated f!(y, x)
800

I suspect that this will fail altogether on the GPU.

[N5N3]

#43322 would take care of this.

[charleskawczynski]

This closed for 1.11, but is still an issue on 1.10.