MethodError when parsing JSON with nested Union types in Tuple field

[srMemphis]

---

title: "MethodError when parsing JSON with nested Union types in Tuple field"

Description

When attempting to parse JSON into a struct containing a Tuple with complex nested Union types, JSON.jl fails with a MethodError about converting Vector{Any} to Union{Float64, Vector{Float64}}.

Minimum Working Example

using JSON

struct foo
    params::Tuple{Union{Float64, Nothing}, Union{Vector{Float64}, Float64}, Union{Vector{Float64}, Float64, Nothing}}
end

# This fails
JSON.parse("""{"params":[null, [1.0, 2.0, 3.0], null]}""", foo)

Error

ERROR: LoadError: MethodError: Cannot `convert` an object of type 
  Vector{Any} to an object of type
  Union{Float64, Vector{Float64}}
The function `convert` exists, but no method is defined for this combination of argument types.

Expected Behavior
The JSON should successfully parse into a foo instance, similar to how JSON3.jl handles this case:

using JSON3

struct foo
    params::Tuple{Union{Float64, Nothing}, Union{Vector{Float64}, Float64}, Union{Vector{Float64}, Float64, Nothing}}
end

# This works in JSON3.jl
JSON3.read("""{"params":[null, [1.0, 2.0, 3.0], null]}""", foo)
# Expected result: foo((nothing, [1.0, 2.0, 3.0], nothing))

Environment
JSON.jl version: 1.4.0
Julia version: 1.12.3