Fix assert fail when fetching pos clobbers ref with undef

pos($x) returns a special magical scalar that sets the match position
on $x.  Calling pos($x) twice will provide two such scalars.  If we
set one of them to a reference, set the other to undef, and then read
the first, all hail breaks loose, because of the use of SvOK_off.

SvOK_off is not sufficient if arbitrary values can be assigned by Perl
code.  Globs, refs and regexps (among others) need special handling,
which sv_setsv knows how to do.

1 files changed, 10 insertions, 1 deletions

diff --git a/t/op/pos.t b/t/op/pos.t
index e9c863b5e4..04b527246d 100644
--- a/t/op/pos.t
+++ b/t/op/pos.t
@@ -6,7 +6,7 @@ BEGIN {
     require './test.pl';
 }
 
-plan tests => 28;
+plan tests => 29;
 
 $x='banana';
 $x=~/.a/g;
@@ -122,3 +122,12 @@ $x =~ /.(?{
     })/;
     is $w, undef, 'and no malformed utf8 warning';
 }
+
+for my $one(pos $x) {
+    for my $two(pos $x) {
+        $one = \1;
+        $two = undef;
+        is $one, undef,
+           'no assertion failure when getting pos clobbers ref with undef';
+    }
+}