#include <functional>
#include <string>

struct Foo {
    Foo(int) {}
    Foo(std::string) {}
    operator int () const { return 42; }
    operator std::string () const { return ""; }

    void foo(std::function<void(Foo&, int)>f);
    void foo(std::function<void(const Foo&, int)>f) const; // xxx

    void foo(std::function<void(const std::string&, Foo&)>f);
    void foo(std::function<void(const std::string&, const Foo&)>f) const;

    void bar() const {
        this->foo([](const Foo&, int){}); // xxx
    }
};

g++ -std=c++11 -c -Wall amb.cpp
amb.cpp: In member function ‘void Foo::bar() const’:
amb.cpp:18:40: error: call of overloaded ‘foo(Foo::bar() const::<lambda(const Foo&, int)>)’ is ambiguous
         this->foo([](const Foo&, int){});
                                        ^
amb.cpp:12:10: note: candidate: void Foo::foo(std::function<void(const Foo&, int)>) const
     void foo(std::function<void(const Foo&, int)>f) const;
          ^
amb.cpp:15:10: note: candidate: void Foo::foo(std::function<void(const std::basic_string<char>&, const Foo&)>) const
     void foo(std::function<void(const std::string&, const Foo&)>f) const;
          ^

#include <functional>

void foo(std::function<void(int)>f);
void foo(std::function<void(double)>f);

void bar() {
    foo([](int){});
}

amb2.cpp: In function ‘void bar()’:
amb2.cpp:8:18: error: call of overloaded ‘foo(bar()::<lambda(int)>)’ is ambiguous
     foo([](int){});
                  ^
amb2.cpp:4:6: note: candidate: void foo(std::function<void(int)>)
 void foo(std::function<void(int)>f);
      ^
amb2.cpp:5:6: note: candidate: void foo(std::function<void(double)>)
 void foo(std::function<void(double)>f);
      ^

this->foo(std::function<void(const Foo&, int)>([](const Foo&, int){}));