Failure to save struct parameterized on Union containing UnionAlls.

[KristofferC]

Reported in KristofferC/NearestNeighbors.jl#71 (comment).

MWE:

using JLD2
struct Foo end
struct Bar{T} end
struct Baz{T <: Union{Foo, Bar}} end
@save "foo.jld2" Baz

gives:

ERROR: MethodError: Cannot `convert` an object of type Type{Bar} to an object of type DataType
Closest candidates are:
  convert(::Type{T}, ::T) where T at essentials.jl:154
Stacktrace:
 [1] setindex!(::Array{DataType,1}, ::Type, ::Int64) at ./array.jl:769
 [2] copyto!(::IndexLinear, ::Array{DataType,1}, ::IndexLinear, ::Array{Any,1}) at ./abstractarray.jl:731
 [3] copyto! at ./abstractarray.jl:723 [inlined]
 [4] Type at ./array.jl:497 [inlined]
 [5] h5convert!(::JLD2.IndirectPointer, ::Type{JLD2.Vlen{JLD2.OnDiskRepresentation{(0, 16),Tuple{String,Array{Any,1}},Tuple{JLD2.Vlen{String},JLD2.Vlen{JLD2.RelOffset}}}()}}, ::JLD2.JLDFile{JLD2.MmapIO}, ::Union, ::JLD2.JLDWriteSession{Dict{UInt64,JLD2.RelOffset}}) at /Users/kristoffer/.julia/packages/JLD2/KjBIK/src/data.jl:964

Seems like a Vector{DataType} is allocated and then the UnionAll is assigned to that which doesn't work.