RT #67962: $1 treated as tainted in untainted match

Fix the issue in the following:

    use re 'taint';
    $tainted =~ /(...)/;
    # $1 now correctly tainted
    $untainted =~ s/(...)/$1/;
    # $untainted now incorrectly tainted

The problem stems from when $1 is updated.

pp_substcont, which is called after the replacement expression has been
evaluated, checks the returned expression for taintedness, and if so,
taints the variable being substituted. For a substitution like
s/(...)/x$1/ this works fine: the expression "x".$1 causes $1's get magic
to be called, which sets $1 based on the recent match, and is marked as
not tainted.  Thus the returned expression is untainted. In the variant
s/(...)/$1/, the returned value on the stack is $1 itself, and its get
magic hasn't been called yet. So it still has the tainted flag from the
previous pattern.

The solution is to mg_get the returned expression *before* testing for
taintedness.

pp_ctl.c

@@ -278,9 +278,11 @@ PP(pp_substcont)
 	if (cx->sb_iters > cx->sb_maxiters)
 	    DIE(aTHX_ "Substitution loop");
 
+	SvGETMAGIC(TOPs); /* possibly clear taint on $1 etc: #67962 */
+
 	if (!(cx->sb_rxtainted & 2) && SvTAINTED(TOPs))
 	    cx->sb_rxtainted |= 2;
-	sv_catsv(dstr, POPs);
+	sv_catsv_nomg(dstr, POPs);
 	/* XXX: adjust for positive offsets of \G for instance s/(.)\G//g with positive pos() */
 	s -= RX_GOFS(rx);
 

t/op/taint.t

@@ -17,7 +17,7 @@ use Config;
 use File::Spec::Functions;
 
 BEGIN { require './test.pl'; }
-plan tests => 321;
+plan tests => 325;
 
 $| = 1;
 
@@ -1380,6 +1380,22 @@ foreach my $ord (78, 163, 256) {
 }
 
 
+# Bug RT #67962: old tainted $1 gets treated as tainted
+# in next untainted # match
+
+{
+    use re 'taint';
+    "abc".$TAINT =~ /(.*)/; # make $1 tainted
+    ok(tainted($1), '$1 should be tainted');
+
+    my $untainted = "abcdef";
+    ok(!tainted($untainted), '$untainted should be untainted');
+    $untainted =~ s/(abc)/$1/;
+    ok(!tainted($untainted), '$untainted should still be untainted');
+    $untainted =~ s/(abc)/x$1/;
+    ok(!tainted($untainted), '$untainted should yet still be untainted');
+}
+
 
 # This may bomb out with the alarm signal so keep it last
 SKIP: {