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Showing content from https://timsong-cpp.github.io/cppwp/n4140/namespace.qual below:

[namespace.qual]

For a namespace X and name m, the namespace-qualified lookup set S(X, m) is defined as follows: Let S'(X, m) be the set of all declarations of m in X and the inline namespace set of X ([namespace.def]). If S'(X, m) is not empty, S(X, m) is S'(X, m); otherwise, S(X, m) is the union of S(N_i, m) for all namespaces N_i nominated by using-directives in X and its inline namespace set.

Given X::m (where X is a user-declared namespace), or given ::m (where X is the global namespace), if S(X, m) is the empty set, the program is ill-formed. Otherwise, if S(X, m) has exactly one member, or if the context of the reference is a using-declaration ([namespace.udecl]), S(X, m) is the required set of declarations of m. Otherwise if the use of m is not one that allows a unique declaration to be chosen from S(X, m), the program is ill-formed. [ Example:

int x;
namespace Y {
  void f(float);
  void h(int);
}

namespace Z {
  void h(double);
}

namespace A {
  using namespace Y;
  void f(int);
  void g(int);
  int i;
}

namespace B {
  using namespace Z;
  void f(char);
  int i;
}

namespace AB {
  using namespace A;
  using namespace B;
  void g();
}

void h(){
  AB::g();                                AB::f(1);                                                                                                               AB::f('c');       
  AB::x++;                                                                        AB::i++;                                                                        AB::h(16.8);                                                                                      }

The same declaration found more than once is not an ambiguity (because it is still a unique declaration). For example:

namespace A {
  int a;
}

namespace B {
  using namespace A;
}

namespace C {
  using namespace A;
}

namespace BC {
  using namespace B;
  using namespace C;
}

void f(){
  BC::a++;          }

namespace D {
  using A::a;
}

namespace BD {
  using namespace B;
  using namespace D;
}

void g(){
  BD::a++;          }

Because each referenced namespace is searched at most once, the following is well-defined:

namespace B {
  int b;
}

namespace A {
  using namespace B;
  int a;
}

namespace B {
  using namespace A;
}

void f(){
  A::a++;             B::a++;             A::b++;             B::b++;           }

 — end example ]

During the lookup of a qualified namespace member name, if the lookup finds more than one declaration of the member, and if one declaration introduces a class name or enumeration name and the other declarations either introduce the same variable, the same enumerator or a set of functions, the non-type name hides the class or enumeration name if and only if the declarations are from the same namespace; otherwise (the declarations are from different namespaces), the program is ill-formed. [ Example:

namespace A {
  struct x { };
  int x;
  int y;
}

namespace B {
  struct y { };
}

namespace C {
  using namespace A;
  using namespace B;
  int i = C::x;       int j = C::y;     }

 — end example ]

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