Re^3: ref to read-only alias ... why? (not consistent)

[LanX]

Your right it's not consistent.

But IMHO consistency will most likely break legacy code.

I think that there should at least be a warning.

Since it's not always clear if a variable is an alias, this can cause very hard to detect errors in subs which are meant to modify call-by-reference parameters. (not to be confused with Perl references)

Did you check if there are already older bug-reports regarding this?

UPDATE:

  DB<108> sub inc_a { $_[0]++; return }
 
  DB<109> $x=1; inc_a($x); print $x
2
  DB<110> inc_a(1)
Modification of a read-only value attempted at (eval 13)[/usr/share/pe
+rl/5.10/perl5db.pl:638] line 2.

but

  DB<111> sub inc_b { my $r=\ $_[0]; $$r++; return }
 
  DB<112> $x=1; inc_b($x); print $x
2
  DB<113> inc_b(1)
 
  DB<114>

When passing an aliasing variable instead of 1 it's the same problem. With inc_b non-aliasing vars will increment, but aliases will silently fail to increment.

[ikegami]

Your right it's not consistent. But IMHO consistency will most likely break legacy code.

I think you're saying that because you envision it being consistently broken (always dying) instead of it being consistently fixed (never dying).

There are two ways of fixing it:

• Have literal constants always return new values. That's inefficient.
• Have literal constants return copy-on-write values. There's currently no such mechanism, and that's a lot of work to implement.

Check out this bug that would also be fixed by either of the above two fixes:

for (1..2) {
   for (1..3) { print $_++; } # 123 234
   print "\n";
}

[dk]

Heh, it's a 8-year old (!!!!) bug: https://rt.perl.org/rt3/Public/Bug/Display.html?id=21979 . With resolution that it's indeed a bug ... well, that situation doesn't seem good to me, when noone wants to touch a bug in 8 years. I wonder why.

[LanX]

Thats what I expected.

> I wonder why.

IMHO legacy code would break if you die here.

But a warning would be great.

[dk]

I agree on a warning, it would be a practical thing, after all. And after that, a real fix with 'die' a couple versions later should help the legacy code.

Well anyway, I found the code suggested by ikegami, so would you guys review the patch please?

--- pp.c.0    2012-01-06 01:07:34.511988700 +0100
+++ pp.c    2012-01-06 19:04:06.311428300 +0100
@@ -515,9 +515,11 @@
     SvTEMP_off(sv);
     SvREFCNT_inc_void_NN(sv);
     }
-    else if (SvPADTMP(sv) && !IS_PADGV(sv))
+    else if (SvPADTMP(sv) && !IS_PADGV(sv)) {
+        Perl_ck_warner(aTHX_ packWARN(WARN_MISC),
+         "Implicit copy of a read-only scalar due to aliasing");
         sv = newSVsv(sv);
-    else {
+    } else {
     SvTEMP_off(sv);
     SvREFCNT_inc_void_NN(sv);
     }