two_phase_lookup.cpp

// Two-phase lookup

#include <iostream>

namespace A {
class MyInt {
  public:
    MyInt(int) {}
};

MyInt operator-(MyInt const&) { return MyInt{0}; }
bool operator>(MyInt const&, MyInt const&) { return false; }

// replacing MyInt with int won't work,
// as int does not have an associated namespace to use in ADL
typedef MyInt Int;
} // namespace A

template <typename T>
void f(T i) {
    if (i > 0) {
        // found by ADL at POI second-phase lookup, no need for forward
        // declaration
        g(-i);
    }
}

int main() {
    A::MyInt m(42);
    f(m);
}

namespace A {
// this is NOT found at second-phase lookup, since only ADL is performed
void g() {}

// found by ADL at POI second-phase lookup, no need for forward declaration
void g(Int) { f<Int>(42); }
// Point of instantiation (POI) is here
} // namespace A