Use `@ViewBuilder` in extensions. Basically cast to `AnyView`.

ViewBuilder.swift

extension ButtonStyle where Self == InputFieldButtonStyle<AnyView> {
    internal static func input<Content: View>(mimicksDropdown: Bool = false,
                               @ViewBuilder placeholderContent: @escaping () -> Content? = { nil }) -> InputFieldButtonStyle<AnyView> {
        InputFieldButtonStyle(mimicksDropdown: mimicksDropdown,
                              placeholderContent: {
                    AnyView(placeholderContent())
            })
    }
}