error: expected ')' for ternary condition

I am writing the following ternary condition in my code:

auto shift = 
    (auto diff1 = setPositions.back() - setPositions.front()) ==
    (auto diff2 = posM - posN) ?
        diff1 :
        diff1 > diff2 ?
        diff2 - (diff1 - diff2) :
        diff1 + (diff2 - diff1);

The above code appears within the constructor definition. The class declaration is as given below:

typedef int32_t THIRTY_TWO_BIT_INT;

class setBits{

    public:
        setBits(THIRTY_TWO_BIT_INT, THIRTY_TWO_BIT_INT, 
                unsigned short, unsigned short
               );
        ~setBits();
    private:
        THIRTY_TWO_BIT_INT valA;
        THIRTY_TWO_BIT_INT valB;
        unsigned short posN;
        unsigned short posM;
        THIRTY_TWO_BIT_INT result;
        vector<int>setPositions;
        void calcSetPositions(THIRTY_TWO_BIT_INT&);
        THIRTY_TWO_BIT_INT bm;
        void calcBitMask();
        void remRedSetBits(THIRTY_TWO_BIT_INT&);

};

For the above ternary condition code, I get the following compilation error:

1 In file included from main.cpp:1:
   2 ./code.hpp:42:15: error: expected ')'
   3         (auto diff1 = setPositions.back() - setPositions.front()) ==
   4               ^
   5 ./code.hpp:42:9: note: to match this '('
   6         (auto diff1 = setPositions.back() - setPositions.front()) ==
   7         ^

Can anyone point out the cause of this error?