set magic on $lex for $lex = (index(...) == -1) and make it an lvalue

related to #17737 and fixes #17739

Author: tonycoz

pp.c

@@ -3522,9 +3522,11 @@ PP(pp_index)
     if (PL_op->op_private & OPpTRUEBOOL) {
         PUSHs( ((retval != -1) ^ cBOOL(PL_op->op_private & OPpINDEX_BOOLNEG))
                     ? &PL_sv_yes : &PL_sv_no);
-        if (PL_op->op_private & OPpTARGET_MY)
+        if (PL_op->op_private & OPpTARGET_MY) {
             /* $lex = (index() == -1) */
             sv_setsv(TARG, TOPs);
+            SETTARG;
+        }
     }
     else
         PUSHi(retval);

t/op/index.t

@@ -8,7 +8,7 @@ BEGIN {
 }
 
 use strict;
-plan( tests => 412 );
+plan( tests => 414 );
 
 run_tests() unless caller;
 
@@ -332,4 +332,30 @@ sub run_tests {
 
     }
 
+    {
+        my $store = 100;
+        package MyTie {
+            require Tie::Scalar;
+            our @ISA = qw(Tie::StdScalar);
+            sub STORE {
+                my ($self, $value) = @_;
+
+                $store = $value;
+            }
+        };
+        my $x;
+        tie $x, "MyTie";
+        $x = (index("foo", "o") == -1);
+        ok(!$store, 'magic called on $lexical = (index(...) == -1)');
+    }
+    {
+        is(eval <<'EOS', "a", 'optimized $lex = (index(...) == -1) is an lvalue');
+my $y = "foo";
+my $z = "o";
+my $x;
+($x = (index($y, $z) == -1)) =~ s/^/a/;
+$x;
+EOS
+    }
+
 } # end of sub run_tests