14 Member access control [class.access]

14.6 Multiple access [class.paths]

1
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If a name can be reached by several paths through a multiple inheritance graph, the access is that of the path that gives most access.
[ Example
: 
class W { public: void f(); };
class A : private virtual W { };
class B : public virtual W { };
class C : public A, public B {
  void f() { W::f(); }          // OK
};
Since W​::​f() is available to C​::​f() along the public path through B, access is allowed.
— end example
 ]