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

Section: 25.10.14 [numeric.ops.gcd], 25.10.15 [numeric.ops.lcm] Status: C++17 Submitter: Marshall Clow Opened: 2016-12-16 Last modified: 2020-09-06

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 25.10.14 [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]

2. Edit 25.10.15 [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>`.