20 Library introduction [library]

20.5 Library-wide requirements [requirements]

20.5.5 Conforming implementations [conforming]

20.5.5.6 Constexpr functions and constructors [constexpr.functions]

This International Standard explicitly requires that certain standard library functions are 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.