20 Library introduction [library]

20.5 Library-wide requirements [requirements]

20.5.5 Conforming implementations [conforming] Constexpr functions and constructors [constexpr.functions]

This document explicitly requires that certain standard library functions are constexpr ([dcl.constexpr]).
An implementation shall not declare any standard library function signature as constexpr except for those where it is explicitly required.
Within any header that provides any non-defining declarations of constexpr functions or constructors an implementation shall provide corresponding definitions.