[Bug preprocessor/84864] New: Issues with large line numbers >= 2^31

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84864

            Bug ID: 84864
           Summary: Issues with large line numbers >= 2^31
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: deferred, diagnostic
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dmalcolm at gcc dot gnu.org
  Target Milestone: ---

PR 84852 describes an ICE on this source code:

$ cat test.c
#line 7777777777
int foo (void) { return strlen(""); }

On fixing the ICE, the #line directive leads to negative line numbers, with the
diagnostic reported here:

  test.c:-812156815:25:

The 7777777777 is truncated to unsigned 32 bits (linenum_type), from
0x1cf977871 to 0xcf977871 == 3482810481.

In some places we use linenum_type (unsigned int); in others we use int,
leading to the printing of -812156815 for the line number.

We already warn for too-large line numbers with -pedantic:

test.c:1:7: warning: line number out of range
 #line 7777777777
       ^~~~~~~~~~

There seem to be at least two issues here:
* silent truncation of #line
* use of negative numbers for such cases

I experimented with using linenum_type throughout for the fix for PR 84852, but
it was more invasive that I'd have liked in stage 4, so filing this to track
addressing this when stage 1 opens.