[perl #127877] Emit undef warning on sassign+concat

Code like this:

    my $x;
    $x .= 'a';

is specifically exempted from "use of uninitialized value" warnings,
according to the "Declarations" section of perlsyn, to allow the idiom of
building up a value piecemeal. The same is true of the += and -= operators.
However, breaking the combined assignment up into the underlying operator
and a simple assignment, as in this code:

    my $x;
    $x = $x . 'a';

*should* produce a warning.

That warning was correctly being emitted for addition and subtraction, but
concatenation was behaving as the ".=" case, because "$x = $x . EXPR" is
optimized to the equivalent of "$x .= EXPR".

So we now explicitly detect this case, and emit the desired warning.

pp_hot.c

@@ -284,8 +284,11 @@ PP(pp_concat)
     }
     else { /* $l .= $r   and   left == TARG */
 	if (!SvOK(left)) {
-	    if (left == right && ckWARN(WARN_UNINITIALIZED)) /* $l .= $l */
-		report_uninit(right);
+            if ((left == right                          /* $l .= $l */
+                 || (PL_op->op_private & OPpTARGET_MY)) /* $l = $l . $r */
+                && ckWARN(WARN_UNINITIALIZED)
+                )
+                report_uninit(left);
 	    sv_setpvs(left, "");
 	}
         else {

t/lib/warnings/9uninit

@@ -2138,3 +2138,24 @@ Use of uninitialized value $i in array element at - line 12.
 Use of uninitialized value $k in hash element at - line 12.
 Use of uninitialized value $i in array element at - line 13.
 Use of uninitialized value $k in hash element at - line 13.
+########
+# perl #127877
+use warnings 'uninitialized';
+my ($p, $q, $r, $s, $t, $u, $v, $w, $x, $y);
+$p = $p . "a";
+$q .= "a";
+$r = $r + 17;
+$s += 17;
+$t = $t - 17;
+$u -= 17;
+use integer;
+$v = $v + 17;
+$w += 17;
+$x = $x - 17;
+$y -= 17;
+EXPECT
+Use of uninitialized value $p in concatenation (.) or string at - line 4.
+Use of uninitialized value $r in addition (+) at - line 6.
+Use of uninitialized value $t in subtraction (-) at - line 8.
+Use of uninitialized value $v in integer addition (+) at - line 11.
+Use of uninitialized value $x in integer subtraction (-) at - line 13.