[Bug c/70952] New: Missing warning for likely-erroneous octal escapes in string literals

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70952

            Bug ID: 70952
           Summary: Missing warning for likely-erroneous octal escapes in
                    string literals
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amonakov at gcc dot gnu.org
  Target Milestone: ---

GCC doesn't warn for:

const char s[] = "\008";

(just the two zeros following the backslash become a part of the octal literal,
so the string literal is equivalent to "\0""8")

\008 and \009 in string literals are most likely errors (\08 and \09 work just
as well if a nil character followed by a digit was really intended)

I think a bit of a bikeshed is possible on the point how far we want to take it
(do we warn for "\08"? for "\799"?).  I think warning when the octal escape
with less than 3 digits is followed by [89] is desirable (this catches all of
the above), but warning when octal escape already has 3 digits may be not (this
exempts "\0009" from the warning).