Make empty Size() type stable

[jipolanco]

The Size() constructor (used with no arguments) is currently type unstable, at least on Julia 1.10:

julia> @code_warntype Size()
MethodInstance for Size()
  from Size(s::Union{StaticArraysCore.Dynamic, Int64}...) @ StaticArraysCore ~/.julia/packages/StaticArraysCore/P6PH7/src/StaticArraysCore.jl:476
Arguments
  #self#::Type{Size}
  s::Tuple{}
Body::Size   # <-- in red
1 ─      nothing
│   %2 = Core.apply_type(StaticArraysCore.Size, s)::Type{Size{_A}} where _A   # <-- in red
│   %3 = (%2)()::Size   # <-- in red
└──      return %3

This doesn't seem to be related to the @pure annotations in the Size constructors.

This PR fixes this issue by explicitly creating a Size() constructor which is type stable.

---

For context, this seems to fix an obscure type stability issue I identified when broadcasting a Number and a SVector, which only manifests when Julia is launched with --check-bounds=no. This looks similar to JuliaArrays/StaticArrays.jl#1155, but I'm not sure it's exactly the same issue.

As an example:

using StaticArrays
x = 0.3
y = SVector(0.1, 0.2, 0.3)
@code_warntype x .+ y

On Julia 1.10.4 started with default flags everything looks fine:

julia> @code_warntype x .+ y
MethodInstance for (::var"##dotfunction#225#1")(::Float64, ::SVector{3, Float64})
  from (::var"##dotfunction#225#1")(x1, x2) @ Main none:0
Arguments
  #self#::Core.Const(var"##dotfunction#225#1"())
  x1::Float64
  x2::SVector{3, Float64}
Body::SVector{3, Float64}
1 ─ %1 = Base.broadcasted(Main.:+, x1, x2)::Base.Broadcast.Broadcasted{StaticArraysCore.StaticArrayStyle{1}, Nothing, typeof(+), Tuple{Float64, SVector{3, Float64}}}
│   %2 = Base.materialize(%1)::SVector{3, Float64}
└──      return %2

When Julia is launched with --check-bounds=no:

julia> @code_warntype x .+ y
MethodInstance for (::var"##dotfunction#225#1")(::Float64, ::SVector{3, Float64})
  from (::var"##dotfunction#225#1")(x1, x2) @ Main none:0
Arguments
  #self#::Core.Const(var"##dotfunction#225#1"())
  x1::Float64
  x2::SVector{3, Float64}
Body::Any
1 ─ %1 = Base.broadcasted(Main.:+, x1, x2)::Base.Broadcast.Broadcasted{StaticArraysCore.StaticArrayStyle{1}, Nothing, typeof(+), Tuple{Float64, SVector{3, Float64}}}
│   %2 = Base.materialize(%1)::Any
└──      return %2

The culprit can be tracked down to be the type unstable Size() constructor. Note that Size() is also unstable without --check-bounds=no, but in that case Julia is somehow able to determine the correct return type of the broadcast.

[mateuszbaran]

Thanks, LGTM. Could you just bump patch version?

[jipolanco]

Done!