Citation:
2 The sequences in both forms of header names are mapped in an implementation-defined manner to headers or external source file names as specified in 6.10.2
3 If the characters ', \, ", //, or /* occur in the sequence between the < and > delimiters, the behavior is undefined. Similarly, if the characters ', \, //, or /* occur in the sequence between the " delimiters, the behavior is undefined.(69) A header name preprocessing token is recognized only within a #include preprocessing directive.
....
(69 Thus, sequences of characters that resemble escape sequences cause undefined behavior.