It is unspecified whether any member functions in the C++ standard library are defined as inline ([dcl.fct.spec]).
An implementation may declare additional non-virtual member function signatures within a class:
by adding arguments with default values to a member function signature;188 [ Note: An implementation may not add arguments with default values to virtual, global, or non-member functions. — end note ]
by replacing a member function signature with default values by two or more member function signatures with equivalent behavior; and
by adding a member function signature for a member function name.
A call to a member function signature described in the C++ standard library behaves as if the implementation declares no additional member function signatures.189
Hence, the address of a member function of a class in the C++ standard library has an unspecified type.
A valid C++ program always calls the expected library member function, or one with equivalent behavior. An implementation may also define additional member functions that would otherwise not be called by a valid C++ program.