[Bug c/110654] New: inconsistent error message in presence of unexpected encoded characters

["drepper.fsp+rhbz at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110654

            Bug ID: 110654
           Summary: inconsistent error message in presence of unexpected
                    encoded characters
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: drepper.fsp+rhbz at gmail dot com
  Target Milestone: ---

Take this code which in a similar form was taken from a text document where the
smart quote handling used the U201c and U201d characters instead of simple
ASCII double quotes.  Note, this text should be encoded in UTF-8 and the
environment of the compiler must use UTF-8 as well.

#include <stdio.h>
int main() {
  puts(“hello world”);
  return 0;
}

Compiling this with a recent gcc 13 or older versions leads to these error
messages:

u.c: In function ‘main’:
u.c:3:8: error: stray ‘\342’ in program
    3 |   puts(<U+201C>hello world<U+201D>);
      |        ^~~~~~~~
u.c:3:9: error: ‘hello’ undeclared (first use in this function); did you mean
‘ftello’?
    3 |   puts(“hello world”);
      |         ^~~~~
      |         ftello
u.c:3:9: note: each undeclared identifier is reported only once for each
function it appears in
u.c:3:14: error: expected ‘)’ before ‘world’
    3 |   puts(“hello world”);
      |       ~      ^~~~~~
      |              )
u.c:3:20: error: stray ‘\342’ in program
    3 |   puts(<U+201C>hello world<U+201D>);
      |                           ^~~~~~~~

The problem is the initial message about "stray ‘\342’" and the notation used
in the context line.  In the later the byte is correctly recognized as being
part on an UTF-8 character.

Note that this is in contrast to the C++ frontend which handles this correctly.
 It shows:

u.c:3:8: error: extended character “ is not valid in an identifier
    3 |   puts(“hello world”);
      |        ^

The C frontend should adopt the same code as the C++ frontend.