```
template <class M, class N>
constexpr common_type_t<M,N> gcd(M m, N n);
```

Requires: |m| and |n| shall be representable as a value of common_type_t<M, N>. [ Note: These requirements ensure, for example, that gcd(m, m) = |m| is representable as a value of type M. — end note ]

Remarks: If either M or N is not an integer type, or if either is cv bool, the program is ill-formed.

Returns: Zero when m and n are both zero. Otherwise, returns the greatest common divisor of |m| and |n|.