warn on $a.$b.$c in void context

Zefram · Zefram · commit 3d033384b201 · 2017-12-14T04:23:58.000Z
$a.$b.$c gets transformed early on to execute as ($a.$b).=$c, which didn't warn about void context becuase .= looks like a useful side effect. Happily, the recently-added OPpCONCAT_NESTED flag identifies that this has happened. Make scalarvoid() pay attention to this flag when a concat op is put into void context. Fixes [perl #6997]
diff --git a/op.c b/op.c
@@ -1949,6 +1949,11 @@ Perl_scalarvoid(pTHX_ OP *arg)
             if (o->op_type == OP_REPEAT)
                 scalar(cBINOPo->op_first);
             goto func_ops;
+	case OP_CONCAT:
+            if ((o->op_flags & OPf_STACKED) &&
+		    !(o->op_private & OPpCONCAT_NESTED))
+                break;
+	    goto func_ops;
         case OP_SUBSTR:
             if (o->op_private == 4)
                 break;
diff --git a/t/lib/warnings/op b/t/lib/warnings/op
@@ -2065,3 +2065,12 @@ use warnings;
 $_="3.14159";
 tr/0-9/\x{6F0}-\x{6F9}/;
 EXPECT
+########
+# Useless use of concatenation should appear for any number of args
+use warnings;
+($a, $b, $c) = (42)x3;
+$a.$b;
+$a.$b.$c;
+EXPECT
+Useless use of concatenation (.) or string in void context at - line 4.
+Useless use of concatenation (.) or string in void context at - line 5.
