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

在短格式中使用概念时,如何检索内部类型？

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;
}