如何从概念中检索类型?

假设我有一个概念:

template < typename Group  > concept bool GGroup =
    requires() { typename Group::Inner; };

如何检索类型 Inner 当使用简短形式的概念时?

void doSomething(const GGroup& group)
{
    // an ugly alternative
    using Inner = typename std::decay_t<decltype(group)>::Inner;

    //// could be something like:
    // using Inner = GGroup::Inner;
    // or
    // using Inner = underlyingtype(GGroup)::Inner;
}