Following the Czech song from Eurovision 2019 in Tel-Aviv
It is known that in C++ a friend of a friend is not (automatically) a friend.
Clang however differs on the following code with GCC and MSVC:
class A {
public:
// forward declaration
class Inner2;
private:
class Inner1 {
char foo;
friend class Inner2;
};
public:
class Inner2 {
Inner1 i;
public:
bool operator==(Inner2 other) {
return i.foo == other.i.foo; // OK by GCC, Clang and MSVC++
}
friend bool operator!=(Inner2 a, Inner2 b) {
return a.i.foo != b.i.foo; // Clang accepts, GCC and MSVC++ reject
}
};
};
Code: https://godbolt.org/z/rn48PTe1Y
Which one is correct? If Clang is wrong by being too permissive, what is the best way to allow access (other than providing a public getter?)
A note: if the friend function is just declared in the class and implemented outside, both Clang and GCC reject the code.
It seems to be a known defect in clang, bug id #11515, reported already in 2011, but still not fixed, apparently.
An even simpler example that compiles, and shouldn't (from the bug report above):
class A {
int n;
friend struct B;
};
struct B {
friend int get(A &a) {
return a.n; // clang accepts, and should reject
}
};