[Bug c/91093] New: Error on implicit int by default

[fw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91093

            Bug ID: 91093
           Summary: Error on implicit int by default
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fw at gcc dot gnu.org
  Target Milestone: ---

Once configure scripts have been cleaned up (and we have verified this for
Fedora and perhaps Debian), c99/gnu99/c11/gnu11 modes should all default to
emitting errors for implicit ints.

Implicit ints were removed from C99, but have been kept in GCC so far, mainly
for compatibility with existing configure checks (which give wrong results if
implicit ints are used in tests).

(Support for implicit ints is incompatible with using the auto keyword for
localized type inference, should that ever come for C, so it makes sense to
clean this up.)