I’m trying to understand how to implement `rand`

for custom types. After looking at the documentation, I’ve come up with the following:

```
using Random
struct A{T}
x::T
end
Random.rand(rng::AbstractRNG, ::Type{A{T}}) where T = A(rand(rng, T))
```

This generates single elements as expected:

```
julia> rand(A{Bool})
A{Bool}(false)
```

I thought that generating random vectors would work automatically, but it doesn’t:

```
julia> rand(A{Bool}, 3)
ERROR: MethodError: no method matching Random.Sampler(::Type{TaskLocalRNG}, ::Random.SamplerType{A{Bool}}, ::Val{1})
Closest candidates are:
Random.Sampler(::Type{<:AbstractRNG}, ::Random.Sampler, ::Union{Val{1}, Val{Inf}})
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:147
Random.Sampler(::Type{<:AbstractRNG}, ::Any, ::Union{Val{1}, Val{Inf}})
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:183
Random.Sampler(::Type{<:AbstractRNG}, ::BitSet, ::Union{Val{1}, Val{Inf}})
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/generation.jl:450
...
Stacktrace:
[1] Random.Sampler(T::Type{TaskLocalRNG}, sp::Random.SamplerType{A{Bool}}, r::Val{1})
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:147
[2] Random.Sampler(rng::TaskLocalRNG, x::Random.SamplerType{A{Bool}}, r::Val{1})
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:139
[3] rand(rng::TaskLocalRNG, X::Random.SamplerType{A{Bool}})
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:255
[4] rand!
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:273 [inlined]
[5] rand!
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:269 [inlined]
[6] rand
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:290 [inlined]
[7] rand
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:291 [inlined]
[8] rand(::Type{A{Bool}}, ::Int64)
@ Random /usr/local/julia-1.10.0-rc1/share/julia/stdlib/v1.10/Random/src/Random.jl:294
[9] top-level scope
@ REPL[5]:1
```

I find this particularly surprising because the first closest candidate displayed above should match in my opinion because

```
julia> TaskLocalRNG <: AbstractRNG, Random.SamplerType{A{Bool}} <: Random.Sampler
(true, true)
```

(The second should also match, but it’s less specific.) Am I missing anything?