Error broadcasting string getproperty (regression)

[lmiq]

Seems that this works in version 0.4.4, but is broken in v0.5 (see this thread)

julia> struct A
         s::String
       end

julia> using StructArrays

julia> v = StructArray([A("a"),A("b")])
2-element StructArray(::Vector{String}) with eltype A:
 A("a")
 A("b")

julia> get_s(x) = x.s
get_s (generic function with 1 method)

julia> get_s(v[1])
"a"

julia> get_s.(v)
ERROR: DimensionMismatch("destination axes (1:0,) are not compatible with source axes (Base.OneTo(2),)")
Stacktrace:
 [1] throwdm(axdest::Tuple{UnitRange{Int64}}, axsrc::Tuple{Base.OneTo{Int64}})
   @ Base.Broadcast ./broadcast.jl:1060
 [2] copyto!
   @ ./broadcast.jl:972 [inlined]
 [3] copyto!
   @ ./broadcast.jl:936 [inlined]
 [4] copy
   @ ./broadcast.jl:908 [inlined]
 [5] materialize(bc::Base.Broadcast.Broadcasted{StructArrays.StructArrayStyle{Base.Broadcast.DefaultArrayStyle{1}}, Nothing, typeof(get_s), Tuple{StructVector{A, NamedTuple{(:s,), Tuple{Vector{String}}}, Int64}}})
   @ Base.Broadcast ./broadcast.jl:883
 [6] top-level scope
   @ REPL[7]:1

[Arkoniak]

git bisect shows that breaking change was made in 0c4adf6

[piever]

Just figured out what this is while looking into other broadcast issues. Essentially, while materializing broadcast, we check whether the eltype is a struct type or not. Unfortunately, isstructtype(String) returns true, but fieldnames(String) == (). I confess I'm not entirely sure why isstructtype(String) returns true or whether there is a more reliable way to now if something was defined as a struct.

EDIT: it is enough to test that the struct type has at least a field, otherwise it is impossible to meaningfully collect it into a StructArray.