The warning suppression for -Wstringop-truncation looks for
the next statement after a truncating strncpy to see if it
adds a terminating nul.  This only works when the next
statement can be reached using the Gimple statement iterator
which isn't until after gimplification.  As a result, strncpy
calls that truncate their constant argument that are being
folded to memcpy this early get diagnosed even if they are
followed by the nul assignment:

   const char s[] = "12345";
   char d[3];

   void f (void)
   {
     strncpy (d, s, sizeof d - 1);   // -Wstringop-truncation
     d[sizeof d - 1] = 0;
   }

To avoid the warning I propose to defer folding strncpy to
memcpy until the pointer to the basic block the strnpy call
is in can be used to try to reach the next statement (this
happens as early as ccp1).  I'm aware of the preference to
fold things early but in the case of strncpy (a relatively
rarely used function that is often misused), getting
the warning right while folding a bit later but still fairly
early on seems like a reasonable compromise.  I fear that
otherwise, the false positives will drive users to adopt
other unsafe solutions (like memcpy) where these kinds of
bugs cannot be as readily detected.

Tested on x86_64-linux.

Martin

PS There still are outstanding cases where the warning can
be avoided.  I xfailed them in the test for now but will
still try to get them to work for GCC 9.