auto will ignore the top-level const qualifier and reference.
decltype will include top-level const and reference.
Same type
int i;
auto a1 = i; // int
decltype(i) d1; // int
Different type
int i, &j = i;
auto a2 = j; // int
decltype(j) d2; // int &
const int i;
auto a3 = i; // int
decltype(i) d3 = 42; // const int