[Bug web/103877] New: libstdc++ docs give a bad recommendation for printing C++ defines

[blubban at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103877

            Bug ID: 103877
           Summary: libstdc++ docs give a bad recommendation for printing
                    C++ defines
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: web
          Assignee: unassigned at gcc dot gnu.org
          Reporter: blubban at gmail dot com
  Target Milestone: ---

https://gcc.gnu.org/onlinedocs/libstdc++/faq.html#faq.predefined

> You can also run g++ -E -dM - < /dev/null" to display a
> list of predefined macros for any particular installation.

Issue 1: The unmatched quote.

Issue 2: g++ -E defaults to C mode, where _GNU_SOURCE is, in fact, not defined.
The correct command is  g++ -E -dM -xc++ - < /dev/null