SwiftUI ViewRoot unable to change environment

viewroot_variadics_environment.swift

// Working with SwiftUI variadics, i need to set the environment of a child - which seems like is impossible.
// The spy always outputs "fail".

struct Transform<Content: View, Result: View>: View {

    @ViewBuilder let content: Content
    let transformation: (_VariadicView.Children.Element) -> Result

    var body: some View {
        _VariadicView.Tree(
            Transformer(transformation: transformation)
        ) {
            content
        }
    }

    private struct Transformer: _VariadicView.UnaryViewRoot {

        let transformation: (_VariadicView.Children.Element) -> Result

        func body(children: _VariadicView.Children) -> some View {
            ForEach(children) { child in
                transformation(child)
            }
        }
    }
}

struct TestKey: EnvironmentKey {
    static var defaultValue: String = "fail"
}

struct TestKeySpy: View {
    @Environment(\._testKey) private var value
    var body: some View {
        Text(value)
    }
}

extension EnvironmentValues {
    var _testKey: String {
        get { self[TestKey.self] }
        set { self[TestKey.self] = newValue }
    }
}

// Test using the following:
Transform {
    TestKeySpy()
} transformation: {
    $0.environment(\._testKey, "success")
}