16 Library introduction [library]

16.5 Library-wide requirements [requirements]

16.5.4 Constraints on programs [constraints]

16.5.4.3 Reserved names [reserved.names]

16.5.4.3.2 Macro names [macro.names]

A translation unit that includes a standard library header shall not #define or #undef names declared in any standard library header.
A translation unit shall not #define or #undef names lexically identical to keywords, to the identifiers listed in Table 4, to the attribute-tokens described in [dcl.attr], or to the identifiers expects or ensures, except that the names likely and unlikely may be defined as function-like macros ([cpp.replace]).