16 Library introduction [library]

16.4 Library-wide requirements [requirements]

16.4.5 Constraints on programs [constraints]

16.4.5.3 Reserved names [reserved.names]

16.4.5.3.6 User-defined literal suffixes [usrlit.suffix]

Literal suffix identifiers ([over.literal]) that do not start with an underscore are reserved for future standardization.
Literal suffix identifiers that contain a double underscore __ are reserved for use by C++ implementations.