2837. gcd and lcm should support a wider range of input values

Section: 29.8.13 [numeric.ops.gcd], 29.8.14 [numeric.ops.lcm] Status: C++17 Submitter: Marshall Clow Opened: 2016-12-16 Last modified: 2017-07-30

Priority: 0

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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.

  1. Edit 29.8.13 [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]

  2. Edit 29.8.14 [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>.