similar(::StaticArray, T) when !isbitstype(T) should give a SizedArray

[marius311]

The similar(::StaticArray, T) method returns an MArray even if called on an SArray, rightfully so since there would be no reason to call similar if you couldn't mutate the array later.

However, if !isbitstype(T), then the returned MArray is still un-mutable later because of

StaticArrays.jl/src/MArray.jl

Lines 94 to 99 in 64c64b2

	if isbitstype(T)
	GC.@preserve v unsafe_store!(Base.unsafe_convert(Ptr{T}, pointer_from_objref(v)), convert(T, val), i)
	else
	# This one is unsafe (#27)
	# unsafe_store!(Base.unsafe_convert(Ptr{Ptr{Nothing}}, pointer_from_objref(v.data)), pointer_from_objref(val), i)
	error("setindex!() with non-isbitstype eltype is not supported by StaticArrays. Consider using SizedArray.")

So I think the natural thing would be for similar(::StaticArray, T) to return a SizedArray when !isbitstype(T), as basically suggested by that error message.

If people here agree, I can take a stab at a PR, but also I'm happy if someone else that's more familiar with the code-base does it (probably much faster than I can).

FWIW, this would fix FluxML/Zygote.jl#686 where this came up for me.

[andyferris]

It’s possible that SizedArray is competitive with MArray on Julia 1.5. If so, we should just switch it over.

[c42f]

Well I don't think SizedArray will be competitive wrt allocation of temporaries in the current compiler - I don't think the optimizer can stack allocate those. Ideally we would finish #749 but in a way which is actually efficient, given the changes in 1.5.

More immediately, we could dispatch on isbits(eltype(a)) to ensure we return something which is mutable and as efficient as possible within the constraints of the current compiler. That could be either SizedArray or MArray as necessary, with the goal of returning MArray as much as possible?

[jishnub]

Gentle bump, this leads to failure such as

julia> S = Diagonal(SVector{2,Real}(1,2))
2×2 Diagonal{Real, SVector{2, Real}} with indices SOneTo(2)×SOneTo(2):
 1  ⋅
 ⋅  2

julia> S[:, 1]
ERROR: setindex!() with non-isbitstype eltype is not supported by StaticArrays. Consider using SizedArray.
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] setindex!
   @ ~/.julia/packages/StaticArrays/B0HhH/src/MArray.jl:39 [inlined]
 [3] macro expansion
   @ ./multidimensional.jl:903 [inlined]
 [4] macro expansion
   @ ./cartesian.jl:64 [inlined]
 [5] _unsafe_getindex!
   @ ./multidimensional.jl:898 [inlined]
 [6] _unsafe_getindex(::IndexCartesian, ::Diagonal{Real, SVector{2, Real}}, ::Base.Slice{SOneTo{2}}, ::Int64)
   @ Base ./multidimensional.jl:889
 [7] _getindex
   @ ./multidimensional.jl:875 [inlined]
 [8] getindex(::Diagonal{Real, SVector{2, Real}}, ::Function, ::Int64)
   @ Base ./abstractarray.jl:1241
 [9] top-level scope
   @ REPL[20]:1

It would be good to have a SizedArray here