Write empty Vector{Union{}} as [] instead of {}

[krynju]

Fixes #499.

Union{} is a subtype of every type, so Vector{Union{}} <: AbstractVector{<:Pair} holds and empty vectors with element type Union{} dispatched to

StructUtils.arraylike(::JSONStyle, x::AbstractVector{<:Pair}) = false

and were written as JSON objects. This shows up in practice for values parsed by JSON3.jl, which types every empty array as JSON3.Array{Union{}, …}, so a JSON3 → JSON.jl round trip turned [] into {}.

This adds a more specific method:

StructUtils.arraylike(::JSONStyle, ::AbstractVector{Union{}}) = true

Before / after:

julia> JSON.json(Union{}[]), JSON.json([Union{}[]]), JSON.json([:a => 1]), JSON.json(Pair{String,Int}[])
("{}", "[{}]", "{\"a\":1}", "{}")   # before
("[]", "[[]]", "{\"a\":1}", "{}")   # after

Existing Vector{Pair} → object behaviour is unchanged. Added regression tests in test/json.jl.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.58%. Comparing base (4f3dfdc) to head (1e8834e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #500   +/-   ##
=======================================
  Coverage   92.57%   92.58%           
=======================================
  Files           7        7           
  Lines        1927     1928    +1     
=======================================
+ Hits         1784     1785    +1     
  Misses        143      143           

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[krynju]

Self-review notes (now rebased: this PR is the minimal Union{} fix plus tests):

1. Method ambiguity – the new arraylike(::JSONStyle, ::AbstractVector{Union{}}) is ambiguous with StructUtils' arraylike(::StructStyle, ::Selectors.List) for List{Union{}}, the same way the existing AbstractVector{<:Pair} method already is for List{<:Pair}. I first added an arraylike(::JSONStyle, ::List) method here, but writing a List turned out to be broken on master in more ways (List{Any}, which the selector syntax returns, hits an applyeach ambiguity and cannot be written at all; List items skip lower), so that is now its own PR, Write StructUtils.Selectors.List as a JSON array #502, which resolves this ambiguity too. The two PRs are independent.

2. Test breadth – the motivating case is JSON3.Array{Union{}}, a non-Vector AbstractVector. Tests originally only covered Vector{Union{}}, so narrowing the method to ::Vector{Union{}} would have passed CI while regressing the JSON3 round trip. Added a view(Union{}[], 1:0) case.

3. Upstream (StructUtils) – the root cause Union{} <: Pair also makes StructUtils' dictlike(::Type{<:AbstractVector{<:Pair}}) return true for Vector{Union{}} while arraylike is true too, so make builds it via the dict path. Harmless in practice (the vector is always empty, and make is lenient about dict-vs-array sources anyway), and independent of this PR. Opened Make Vector{Union{}} arraylike, not dictlike JuliaServices/StructUtils.jl#79 to make the traits agree.

Locally all testsets pass; the trailing segfault in the juliac --trim compile stage reproduces on unmodified master with Julia 1.12.7, so it is unrelated to this change.