right shifting negative integers is tecnically undefined, this means that 
the implementation used for encoding integers:
is tecnically undefined according to:

> The value of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits 
> are zero-filled. If E1 has an unsigned type, the value of the result is E1 
> × 2 E2, reduced modulo one more than the maximum value representable in the 
> result type. Otherwise, if E1 has a signed type and non-negative value, and 
> E1 × 2 E2 is representable in the corresponding unsigned type of the result 
> type, then that value, converted to the result type, is the resulting 
> value; otherwise, the behavior is undefined.

and the implementation for decoding:
depends on the 2s complement as well and is probably tecnically undefined 
(but I cant find a quote from the standard).

This is why I propose the following solution:

template<typename S,class U = typename std::make_unsigned<S>::type>>
U constexpr zigzag_encode(const S &n){
  return (U(n)<<1)^(n<0?~U(0):U(0));

template<typename U,class S = typename std::make_signed<U>::type>>
S constexpr zigzag_decode(const U &n){
  return (n&1)?-1-S(n>>1):(n>>1);

This does not look as cool, but modern compilers (llvm and gcc were tested) 
will compile to bascially the same instructions, gcc will introduce 1 
additional instruction for decode.

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