Should `LinearAlgebra.fzero` be declared public?

[jishnub]

https://github.com/JuliaLang/julia/blob/5e4669c7403c301985f35d2c8754b184cd73ab05/stdlib/LinearAlgebra/src/structuredbroadcast.jl#L142-L151

This method is useful in general, as it allows custom types to hook into the structure-preserving broadcast mechanism without having to define their own methods for each function. As an example:

julia> using FillArrays, LinearAlgebra

julia> Diagonal(1:4) .+ Zeros(4,4)
4×4 Matrix{Float64}:
 1.0  0.0  0.0  0.0
 0.0  2.0  0.0  0.0
 0.0  0.0  3.0  0.0
 0.0  0.0  0.0  4.0

julia> LinearAlgebra.fzero(x::Zeros) = zero(eltype(x))

julia> Diagonal(1:4) .+ Zeros(4,4)
4×4 Diagonal{Float64, Vector{Float64}}:
 1.0   ⋅    ⋅    ⋅ 
  ⋅   2.0   ⋅    ⋅ 
  ⋅    ⋅   3.0   ⋅ 
  ⋅    ⋅    ⋅   4.0

Perhaps this could also be done through an isstructuredmatrix trait that is made public.

cc: @dlfivefifty @dkarrasch

[dlfivefifty]

What is meant by "declared public"? I think it should be fine if FillArrays.jl uses this now.

[jishnub]

As in, a guarantee that this won't be removed in the next minor release

[dlfivefifty]

StdLib can have breaking changes in 1.x releases so there is no such thing as a "guarantee". Only Base is guaranteed to be non-breaking.

I think we should just use this in FillArrays.jl. I'm about to tag a breaking release