Parametric struct with type of member depending on value type of struct

I'm trying to write a parametric struct where one of the parameters is a Boolean value type, and where the type of a member should depend on the value of this Boolean. The struct should also inherit from a parametric type, where the parameters again depend on the Boolean. Simplified, this looks something like this:

struct Test{D} <: AbstractDict{String, Union{String, D==true ? Number : Union{}}}
    d::Dict{String, Union{String, D==true ? Number : Union{}}}
end

However, when I create an object with Test{true}(Dict{String, Union{String, Number}}()) the type parameter seems to always be ignored and I end up with d::Dict{String, String}.

Is what I'm trying to do even possible in Julia?

Apparently, one way to specify a type V, for both field type and supertype parameters, that depends on a constructor boolean value type parameter D, without specifying V in the constructor type parameter, is to:

• Add struct type parameter V to specify that the value types are the same in supertype and field.
• Specify each boolean value in an inner constructor type parameter to constrain its result type.
• Omit the V in each inner constructor type parameter, and specify its value in each new expression.

struct TestC{D,V} <: AbstractDict{String, V}
    d::AbstractDict{String, V}
    TestC{true}() = new{true, Union{String, Number}}(Dict{String, Union{String, Number}}())
    TestC{false}() = new{false, String}(Dict{String, String}())
    TestC(aDict::S) where S <: AbstractDict{String, Union{String, Number}} = new{true, Union{String, Number}}(aDict)
    TestC(aDict::S) where S <: AbstractDict{String, String} = new{false, String}(aDict)
end

import Base: iterate, length, get
Base.iterate(t::TestC) = iterate(t.d)
Base.iterate(t::TestC, i::Int64) = iterate(t.d, i)
Base.length(t::TestC) = length(t.d)
Base.get(t::TestC, k::String, dflt) = get(t.d, k, dflt)

import Test: @testset, @test
@testset begin
    @test TestC{true, Union{String, Number}} == typeof(TestC{true}())
    @test TestC{false, String} == typeof(TestC{false}())
    @test TestC{true, Union{String, Number}} == typeof(TestC(Dict{String, Union{String, Number}}()))
    @test TestC{false, String} == typeof(TestC(Dict{String, String}()))
    @test "TestC{true, Union{Number, String}}(\"one\" => 1)" == string(TestC(Dict{String, Union{String, Number}}("one" => 1)))
    @test 1 == TestC(Dict{String, Union{String, Number}}("one" => 1))["one"]

    @test TestC{true, Union{String, Number}} <: AbstractDict{String, Union{String, Number}}
    @test TestC{false, String} <: AbstractDict{String, String}
    @test false == (TestC{true, Union{String, Number}} <: AbstractDict{String, String})
    @test false == (TestC{false, String} <: AbstractDict{String, Union{String, Number}})

    @test "TestC{true, Union{Number, String}}(\"one\" => 1)" == string(TestC(TestC(Dict{String, Union{String, Number}}("one" => 1))))
    @test "TestC{false, String}()" == string(TestC(TestC{false}()))
end