Here's another small refinement to -Wstringop-truncation to
avoid diagnosing more arguably "safe" cases of strncat() that
match the expected pattern of
strncat (d, s, sizeof d - strlen (d) - 1);
such as
extern char a[4];
strncat (a, "12", sizeof d - strlen (a) - 1);
Since the bound is derived from the length of the destination
as GCC documents is the expected use, the call should probably
not be diagnosed even though truncation is possible.
The trouble with strncat is that it specifies a single count
that can be (and has been) used to specify either the remaining
space in the destination or the maximum number of characters
to append, but not both. It's nearly impossible to tell for
certain which the author meant, and if it's safe, hence all
this fine-tuning. I suspect this isn't the last tweak, either.
In any event, I'd like to commit the patch to both trunk and
gcc-8-branch. The bug isn't marked regression but I suppose
it could (should) well be considered one.
Martin