This is a valid program:
```
constexpr auto f = [](auto& x){
    x.m();
};

class A{
    friend decltype(f);
    void m() {}
public:
    void n() {
        f(*this);
    }
};

int main(){
    A{}.n();
}
```
which is accepted by Clang and MSVC, but unfortunately not by GCC.

The error is
```
error: 'void A::m()' is private within this context
    2 |     x.m();
```
Demo: https://gcc.godbolt.org/z/vKnhqG9b1

Related discussion: https://stackoverflow.com/q/64390688/7325599