Can you explicitly provide template arguments to std::gcd?

Question

[numeric.ops.gcd] states:

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

Mandates: M and N both are integer types other than cv bool.

While doing so is not very useful, it seems permitted to call:

std::gcd<const int, volatile int>(0, 0);

That is because const int is an arithmetic type. However, [algorithms.requirements] paragraph 15 states:

The well-formedness and behavior of a call to an algorithm with an explicitly-specified template argument list is unspecified, except where explicitly stated otherwise.

This begs the question: Is std::gcd an algorithm, and more generally, what is an algorithm? I was unable to find a definition of "algorithm", and since [numeric.ops.gcd] is nested in [algorithm], it may be considered one.

Answer 1

Yes, std::gcd is an algorithm.

Table 83 (algorithms library summary, reproduced-ish below) lists numeric.ops a.k.a. the <numeric> header as part of the library, and std::gcd falls under it (numeric.ops.gcd)

Subclause	Description	Header
[algorithms.requirements]	Algorithms requirements
[algorithms.parallel]	Parallel algorithms	<execution>
[algorithms.results]	Algorithm result types	<algorithm>
[alg.nonmodifying]	Non-modifying sequence operations
[alg.modifying.operations]	Mutating sequence operations
[alg.sorting]	Sorting and related operations
[numeric.ops]	Generalized numeric operations	<numeric>
[specialized.algorithms]	Specialized <memory> algorithms	<memory>
[alg.rand]	Specialized <random> algorithms	<random>
[alg.c.library]	C library algorithms	<cstdlib>