[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.

1 files changed, 5 insertions, 2 deletions

diff --git a/pp_hot.c b/pp_hot.c
index d6cb1aa091..223169bd53 100644
--- a/pp_hot.c
+++ b/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 {