This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++17 status.
Section: 27.10.14 [numeric.ops.gcd], 27.10.15 [numeric.ops.lcm] Status: C++17 Submitter: Marshall Clow Opened: 2016-12-16 Last modified: 2020-09-06
Priority: 0
View all other issues in [numeric.ops.gcd].
View all issues with C++17 status.
Discussion:
This is a duplicate of 2792, which addressed LFTS 2.
By the current definition, gcd((int64_t)1234, (int32_t)-2147483648) is ill-formed (because 2147483648 is not representable as a value of int32_t.) We want to change this case to be well-formed. As long as both |m| and |n| are representable as values of the common type, absolute values can be calculate d without causing unspecified behavior, by converting m and n to the common type before taking the negation.
Suggested resolution:
|m| shall be representable as a value of type M and |n| shall be representable as a value of type N|m| and |n| shall be representable as a value of common_type_t<M, N>.
[2017-01-27 Telecon]
Priority 0
Proposed resolution:
This wording is relative to N4604.
Edit 27.10.14 [numeric.ops.gcd] as indicated:
template<class M, class N> constexpr common_type_t<M, N> gcd(M m, N n);-2- Requires:
|m| shall be representable as a value of type M and |n| shall be representable as a value of type N|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]
Edit 27.10.15 [numeric.ops.lcm] as indicated:
template<class M, class N> constexpr common_type_t<M, N> lcm(M m, N n);-2- Requires:
|m| shall be representable as a value of type M and |n| shall be representable as a value of type N|m| and |n| shall be representable as a value of common_type_t<M, N>. The least common multiple of |m| and |n| shall be representable as a value of type common_type_t<M, N>.