Fix my $x = 3; $x = length(undef);.

Assignment of length() to a lexical is optimized by passing the
assigned-to variable as TARG, avoiding a pp_padsv and pp_sassign.
9f621b which changed length(undef) to return undef didn't take this into
account, and used SETs (which doesn't set TARG), so the code above left
$x == 3.

1 files changed, 6 insertions, 3 deletions

diff --git a/pp.c b/pp.c
index 0da8bba494..fcb7ff2d97 100644
--- a/pp.c
+++ b/pp.c
@@ -3105,8 +3105,10 @@ PP(pp_length)
 	    = sv_2pv_flags(sv, &len,
 			   SV_UNDEF_RETURNS_NULL|SV_CONST_RETURN|SV_GMAGIC);
 
-	if (!p)
-	    SETs(&PL_sv_undef);
+	if (!p) {
+	    sv_setsv(TARG, &PL_sv_undef);
+	    SETTARG;
+	}
 	else if (DO_UTF8(sv)) {
 	    SETi(utf8_length((U8*)p, (U8*)p + len));
 	}
@@ -3119,7 +3121,8 @@ PP(pp_length)
 	else
 	    SETi(sv_len(sv));
     } else {
-	SETs(&PL_sv_undef);
+	sv_setsv_nomg(TARG, &PL_sv_undef);
+	SETTARG;
     }
     RETURN;
 }