Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment,
-Wtrigraphs, -Wmultichar and a warning about integer promotion causing a change of sign in "#if"
expressions. Note that many of the preprocessor's warnings are on by default and have no options to
Most trigraphs in comments cannot affect the meaning of the program. However, a trigraph that would
form an escaped newline (??/ at the end of a line) can, by changing where the comment begins or ends.
Therefore, only trigraphs that would form escaped newlines produce warnings inside a comment.
This option is implied by -Wall. If -Wall is not given, this option is still enabled unless
trigraphs are enabled. To get trigraph conversion without warnings, but get the other -Wall
warnings, use -trigraphs-Wall-Wno-trigraphs.