s/.../$_++/ge assertion failure

The code that updated pos() on a match string assumed that it was SvPOK().
Cunning code like the following converted $_ to SvIOK only:

    $_ = 0;
    s/.?/$_++/ge;

1 files changed, 10 insertions, 1 deletions

diff --git a/t/re/subst.t b/t/re/subst.t
index 7b9a44b991..6963c4217e 100644
--- a/t/re/subst.t
+++ b/t/re/subst.t
@@ -8,7 +8,7 @@ BEGIN {
     require './charset_tools.pl';
 }
 
-plan( tests => 260 );
+plan( tests => 261 );
 
 $_ = 'david';
 $a = s/david/rules/r;
@@ -1052,3 +1052,12 @@ SKIP: {
 }
 
 }
+
+{
+    # RT #123954 if the string getting matched against got converted during
+    # s///e so that it was no longer SvPOK, an assertion would fail when
+    # setting pos.
+    my $s1 = 0;
+    $s1 =~ s/.?/$s1++/ge;
+    is($s1, "01","RT #123954 s1");
+}