C++ how to check if a template class parameter's member variable is accessible (friendship considered)?

Question

I am trying something similar to this, i.e. checking if an accessible member exists before accessing it:

template<class T> concept IsA_ = requires (T t) { t.a; };

class A {
    friend class Friend;
    bool a;
 };

class Friend {

    template<class T> requires IsA_<T>
    void foo() { T{}.a = true; }

    void bar() { foo<A>(); } <--- "the associated constraints are not satisfied"
};

The code does not compile due to the concept IsA_ not being fulfilled.

Although A's member a is perfectly accessible from within Friend, the concept does not agree.

Is there a way to make this work?

Note: Changing member a to be a public member is not a solution.

Note: Changing member a to be a protected member and then letting Friend inherit from A is not a solution.

Note: Letting IsA_ check for any private member a is not a solution.

Note: Letting IsA_ check if T is of type class A is not a solution (IsA_ refers to a set of properties rather than a fixed structure).