Inconsistent conversion of LazyJSON.Object to Union type

[mstetson]

I'm pretty new to Julia, so I'm not sure whether this is a bug in LazyJSON, a bug in Julia itself, or a bug in my understanding. I'm trying to save some values from a JSON document into a struct type that includes a Union{LazyJSON.Object, Nothing}. Depending on exactly how I call the constructor, Julia may try to call convert to convert the LazyJSON.Object to the union type. If that happens, the method that gets called is an @generated method for LazyJSON's struct conversion, which fails trying to call fieldnames() on the union type.

Here's a demonstration of the problem with LazyJSON 0.1.1 and Julia 1.1.0.

               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.1.0 (2019-01-21)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> import LazyJSON

julia> struct T s::String; u::Union{LazyJSON.Object,Nothing} end

julia> T(LazyJSON.value("\"foo\""), nothing)
T("foo", nothing)

julia> T("foo", LazyJSON.value("{\"foo\":\"bar\"}"))
T("foo", LazyJSON.Object{Nothing,String}("foo"=>"bar"))

julia> T(LazyJSON.value("\"foo\""), LazyJSON.value("{\"foo\":\"bar\"}"))
ERROR: MethodError: no method matching fieldnames(::Type{Union{Nothing, Object}})
Closest candidates are:
  fieldnames(::Core.TypeofBottom) at reflection.jl:175
  fieldnames(::Type{#s21} where #s21<:Tuple) at reflection.jl:177
  fieldnames(::DataType) at reflection.jl:172
  ...
Stacktrace:
 [1] #s71#28(::Any, ::Any, ::Any, ::Any) at /Users/micah/.julia/packages/LazyJSON/EmafJ/src/AbstractDict.jl:82
 [2] (::Core.GeneratedFunctionStub)(::Any, ::Vararg{Any,N} where N) at ./boot.jl:522
 [3] T(::LazyJSON.String{String}, ::LazyJSON.Object{Nothing,String}) at ./REPL[4]:1
 [4] top-level scope at none:0