[perl #108480] $cow |= number undefines $cow

If a read-only scalar is passed to one of | & ^ and it decides to do
a numeric operation, the numeric flags on the read-only scalar are
turned off afterwards if they were not on to begin with.

This was introduced in commit b20c4ee1f, which did so to stop $x | "0"
from coercing the rhs and making it behave differently the second
time through.

What that commit did not take into account was that the read-only
flag is set on cow scalars, and the same pp function is used for the
assignment forms.  So it was turning off the numeric flags after
$cow |= 1, leaving $cow undef.

I made this numeric flag-twiddling apply only to read-only scalars
(supposedly), because that seemed the most conservative and acceptable
change.  I am actually in favour of extending it to all scalars, to
make these operators less surprising.  For that reason, this commit
preserves the current behaviour with cows in the non-assignment case:
they don’t get coerced into numbers.  Changing them to work the same
way as non-cow writable scalars would make things more consistent, but
more consistently buggy.  I would like to make this non-coercion apply
to all scalars in 5.18.

This commit simply skips the flag-twiddling on the lhs in the assign-
ment case.

1 files changed, 2 insertions, 2 deletions

diff --git a/pp.c b/pp.c
index b54b3abc8b..2bc1a1a34f 100644
--- a/pp.c
+++ b/pp.c
@@ -2193,7 +2193,7 @@ PP(pp_bit_and)
 	  const UV u = SvUV_nomg(left) & SvUV_nomg(right);
 	  SETu(u);
 	}
-	if (left_ro_nonnum)  SvNIOK_off(left);
+	if (left_ro_nonnum && left != TARG) SvNIOK_off(left);
 	if (right_ro_nonnum) SvNIOK_off(right);
       }
       else {
@@ -2227,7 +2227,7 @@ PP(pp_bit_or)
 	  const UV result = op_type == OP_BIT_OR ? (l | r) : (l ^ r);
 	  SETu(result);
 	}
-	if (left_ro_nonnum)  SvNIOK_off(left);
+	if (left_ro_nonnum && left != TARG) SvNIOK_off(left);
 	if (right_ro_nonnum) SvNIOK_off(right);
       }
       else {