[Bug c/81851] New: missing -Wduplicated-branches on if and return statements with no else

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81851

            Bug ID: 81851
           Summary: missing -Wduplicated-branches on if and return
                    statements with no else
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The -Wduplicated-branches warning triggers when the true statement and the
false statement in an if statement are the same, including when both are a
return statement.  But the warning doesn't trigger for the equivalent code with
the else statement, even though it's very common to leave out the else part
when the block controlled by the if statement ends in a return.

This could be viewed more as an enhancement request than a bug since the
documentation doesn't suggest a warning in the latter case should be expected,
but since such code is pervasive I'd be more inclined to see it as a bug. 
Either way, handling this case would make the warning obviously even more
useful than it already is.

$ cat b.c && gcc -O2 -S -Wduplicated-branches b.c
int f (int i)
{
  if (i == 0)   // -Wduplicated-branches (good)
    return 0;
  else
    return 0;
}

int g (int i)
{
  if (i == 0)   // no warning
    return 0;
  return 0;
}

b.c: In function ‘f’:
b.c:3:6: warning: this condition has identical branches [-Wduplicated-branches]
   if (i == 0)   // -Wduplicated-branches (good)
      ^