Friend function is not visible in the class

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The friend declaration states that a function called f in the surrounding namespace is a friend of the class; but it does not introduce the name f into the namespace. It’s not available (except by argument-dependent lookup) until it’s been declared in the namespace.

The relevant rule is C++11 7.3.1.2/3:

If a friend declaration in a non-local class first declares a class or function the friend class or function is a member of the innermost enclosing namespace. The name of the friend is not found by unqualified lookup or by qualified lookup until a matching declaration is provided in that namespace scope.

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