[Bug other/81969] New: "el" escape sequence in coloring yields incorrect message in some terminals

["vincent-gcc at vinc17 dot net" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81969

            Bug ID: 81969
           Summary: "el" escape sequence in coloring yields incorrect
                    message in some terminals
           Product: gcc
           Version: 6.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

Consider a tst.c file:

int foo (void)
{
  int x, xx;
  return x + xx;
}

In a 80-column xterm, I get (where | represents the right border of the
terminal):

cventin% gcc-6 -Wall -c tst.c                                                  
|
tst.c: In function ‘foo’:                                                      
|
tst.c:4:12: warning: ‘x’ is used uninitialized in this function
[-Wuninitialize]|
   return x + xx;                                                              
|
          ~~^~~~                                                               
|
tst.c:4:12: warning: ‘xx’ is used uninitialized in this function
[-Wuninitialize|
d]                                                                             
|

i.e. the "d" of -Wuninitialized is missing in the first message.

In a 80-column rxvt, I get:

cventin% gcc-6 -Wall -c tst.c                                                  
|
tst.c: In function ‘foo’:                                                      
|
tst.c:4:12: warning: ‘x’ is used uninitialized in this function [-Wuninitialize
|
]                                                                              
|
   return x + xx;                                                              
|
          ~~^~~~                                                               
|
tst.c:4:12: warning: ‘xx’ is used uninitialized in this function
[-Wuninitialize|
d]                                                                             
|

Here the "d" is replaced by a space.

In the xterm logs, I see a \E[K ("el" escape sequence, which clears to the end
of the line) after the first 80 characters. Thus this may be the same bug as in
grep: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=15444
I quote:

  The EL command (\E[K) is sent to clear the line. The VT100
  terminal autowraps when the 81th character is received and the
  EL command is not counted as a character. So if 80 characters
  are received, it waits on the 80th character and the following
  EL command erases characters from the current one to the end
  of line (i.e. the last character on the line).