g++ -std=gnu++11 1.cpp
1.cpp: In function ‘int main()’:
1.cpp:4:19: error: two consecutive ‘[’ shall only introduce an attribute before ‘[’ token
     std::cout << s[[]{ return 1; }()];
                   ^
1.cpp:4:19: error: expected ‘;’ before ‘[’ token
1.cpp:4:36: error: expected primary-expression before ‘)’ token
     std::cout << s[[]{ return 1; }()];
                                    ^
1.cpp:4:37: error: expected ‘;’ before ‘]’ token
     std::cout << s[[]{ return 1; }()];
                                     ^