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enum class SomeEnum { ONE, TWO, THREE };
 
template<typename Enum>
class Flags {
    Flags(const Flags& f) { };
    Flags(Enum e) { };
 
    Flags& operator |=(Flags<Enum> f) {
        //...;
        return *this;
    }
 
friend Flags operator |(Enum a, Flags b) { return b | a; }
friend Flags operator |(Flags a, Enum b) { return a |= b; }
};
 
 
int main() {
    Flags<SomeEnum> flags = SomeEnum::ONE | SomeEnum::TWO;
    return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:19:43: error: no match for ‘operator|’ (operand types are ‘SomeEnum’ and ‘SomeEnum’)
     Flags<SomeEnum> flags = SomeEnum::ONE | SomeEnum::TWO;
                             ~~~~~~~~~~~~~~^~~~~~~~~~

Standard output is empty