foo.cc:76:33: error: 'operator[]' function uses 'auto' type specifier without trailing return type
    auto& operator[] (const int i) {
                                 ^
foo.cc:76:33: note: deduced return type only available with -std=c++14 or -std=gnu++14
foo.cc:79:41: error: 'operator[]' function uses 'auto' type specifier without trailing return type
    const auto& operator[] (const int i) const {
                                         ^~~~~
foo.cc:79:41: note: deduced return type only available with -std=c++14 or -std=gnu++14

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