[Bug c++/98967] New: warning to spot recursive include graph

[nathan at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98967

            Bug ID: 98967
           Summary: warning to spot recursive include graph
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nathan at gcc dot gnu.org
  Target Milestone: ---

recursive headers can slip in unnoticed (eg the new<...>exception loop found in
2020).  Often these are harmless, but with c++20 header-units they can be
fatal.  They are hard to locate, as absent modules they could be harmless.

Add a warning to spot them in the preprocessor.