zero-length MVector to SVector conversion fails (type info is lost)

[tpapp]

julia> using StaticArrays

julia> m = MVector{0,Int}()
0-element MArray{Tuple{0},Int64,1,0} with indices SOneTo(0)

julia> SVector(m)
ERROR: UndefVarError: T not defined
Stacktrace:
 [1] SArray{Tuple{0},T,1,0} where T(::Tuple{}) at /home/tamas/.julia/packages/StaticArrays/mlIi1/src/SVector.jl:19
 [2] SArray at /home/tamas/.julia/packages/StaticArrays/mlIi1/src/SVector.jl:18 [inlined]
 [3] SArray{Tuple{S},T,1,S} where T where S(::MArray{Tuple{0},Int64,1,0}) at /home/tamas/.julia/packages/StaticArrays/mlIi1/src/convert.jl:5
 [4] top-level scope at REPL[32]:1
 [5] eval(::Module, ::Any) at ./boot.jl:331
 [6] eval_user_input(::Any, ::REPL.REPLBackend) at /home/tamas/src/julia-1.4/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:86
 [7] run_backend(::REPL.REPLBackend) at /home/tamas/.julia/packages/Revise/MgvIv/src/Revise.jl:1023
 [8] top-level scope at none:0

I am happy to make a PR, just let me know what the preferred solution is. Maybe special-case 0-length? Or carry the type in the conversion?

[c42f]

This is one of those somewhat embarrassing issues which has been around for ages and which nobody got around to fixing. I think this is the same as #557, but maybe having two issues open for this will encourage someone to finally fix it!

I'd be very happy to have a PR fixing this. It shouldn't need to specialize the size-0 case, but make sure that T is preserved. Tricky things to watch out for when modifying the constructors are that the resulting internal code works for both types and values, empty and nonempty and for all staticarray subtypes.