17 Library introduction [library]

17.6 Library-wide requirements [requirements]

17.6.4 Constraints on programs [constraints] Headers [alt.headers]

If a file with a name equivalent to the derived file name for one of the C++ standard library headers is not provided as part of the implementation, and a file with that name is placed in any of the standard places for a source file to be included ([cpp.include]), the behavior is undefined.