Unbound type param false positive in `Type{Union{...}}`

[nickrobinson251]

Consider the module:

module Foo
    foo(::Type{Union{Nothing,Vector{T}}}) where T = 1
    foo(::Type{Nothing}) = 2
    foo(::Type{Vector{T}}) where T = 3
end

As far as I can tell, T cannot be unbound in the first definition, because it is only dispatched to by a Union{Nothing,Vector{T}} where T, e.g.

julia> begin
                  using .Foo: foo
                  @show foo(Vector{Int})
                  @show foo(Nothing)
                  @show foo(Union{Nothing,Vector{Float64}}) # only this call can hit the first definition
                  # no other calls would match the signature:
                  @show Type{Nothing} <: Type{Union{Nothing,Vector{T}}} where T
                  @show Type{Vector{Float64}} <: Type{Union{Nothing,Vector{T}}} where T
                  @show Type{Union{Nothing,Vector}} <: Type{Union{Nothing,Vector{T}}} where T
       end
foo(Vector{Int}) = 3
foo(Nothing) = 2
foo(Union{Nothing, Vector{Float64}}) = 1
Type{Nothing} <: (Type{Union{Nothing, Vector{T}}} where T) = false
Type{Vector{Float64}} <: (Type{Union{Nothing, Vector{T}}} where T) = false
Type{Union{Nothing, Vector}} <: (Type{Union{Nothing, Vector{T}}} where T) = false

But Aqua.test_unbound_args says T can be unbound:

julia> Aqua.test_unbound_args(Foo)
Unbound type parameters detected:
[1] foo(::Type{Union{Nothing, Vector{T}}}) where T @ Main.Foo REPL[12]:2
Test Failed at /Users/nickr/.julia/packages/Aqua/EPo21/src/unbound_args.jl:37
  Expression: isempty(unbounds)
   Evaluated: isempty(Method[foo(::Type{Union{Nothing, Vector{T}}}) where T @ Main.Foo REPL[12]:2])
ERROR: There was an error during testing

I think this is a false positive?

(Note this is not the same case as the Type issue in #333 or the Union 'issues' in #265 or #252)

[lgoettgens]

Thanks for your report. Unfortunately, this test is not implemented in Aqua.jl, but we instead call Test.detect_unbound_args from the julia Test stdlib. Could you please open an issue about this in the julialang/julia repository and link it here? Thanks

[nickrobinson251]

JuliaLang/julia#59023 / JuliaLang/julia#58427