The code below fails with the error:

error C2923: 'std::is_same_v': 'A::B' is not a valid template type argument for parameter '<unnamed-symbol>'

#include <iostream>

//Primary template
template<typename T> struct ClassOf {
};

// Partial specialisation
template<typename Return, typename Class>
struct ClassOf<Return(Class::*)> {
    using type = Class;};

//An alias
template< typename T> using ClassOf_t = typename ClassOf<T>::type;

struct A { 
    struct B {
        void methodB () {}

    };
    B* myPtr;
    // Rename the function to BB() and do this as well on L29 and the code will compile
    B* B() { return myPtr; }
};

struct C {
    template <typename T>
    T* getClient() {
        if constexpr (std::is_same_v<A::B,T>) {
            return myA.B();
        }
    }
    A myA;
};

int main()
{
    auto fn = &A::B::methodB;
    ClassOf_t<decltype(fn)> a;
    a.methodB();
    C c;
    auto res = c.getClient<ClassOf_t<decltype(fn)>>();
    res->methodB();
    return 0;
}

However, if I rename the function on L22 to BB():

B* BB() { return myPtr; }

and do this as well on L29:

return myA.BB();

Then the code compiles.

Is there any way I can make this compile without changing anything in struct A?

I would not myself choose member function names that could cause conflict, but I am working with GRPC and those generated client classes look like struct A above.

For example, the link below shows this for the Stub class (on L164) which has an async nested class (L208) and an async() member function (L224):

https://github.com/SafetyCulture/s12-proto/blob/master/protobuf/protoc-gen-cruxclient/generated/routeguide/v1/route_guide.grpc.pb.h