Perl 5.36 warns for magic comparison

[hurricup]

According to the perldebguts:

Each array @{"_<$filename"} holds the lines of $filename for a file compiled by Perl. The same is also true for evaled strings that contain subroutines, or which are currently being executed. The $filename for evaled strings looks like (eval 34).
Values in this array are magical in numeric context: they compare equal to zero only if the line is not breakable.

But perl 5.36 warns in my debugger as: Argument "use v5.36;\n" isn't numeric in numeric eq (==) at
There is a simple comparison: if $lines[$lineno] == 0
Do I do something wrong?

[ikegami]

Using

#!/usr/bin/perl

use strict;
use warnings;

use v5.36;

say 'Hello, world!';

In 5.38, we see:

  DB<1> use Devel::Peek;

  DB<2> Dump( ${"_<a.pl"}[8] );
SV = PVMG(0x55d8627958e0) at 0x55d86251ff80
  REFCNT = 1
  FLAGS = (IOK,POK,IsCOW,pIOK,pPOK)
  IV = 94387850872920
  NV = 0
  PV = 0x55d8628074b0 "say 'Hello, world!';\n"\0
  CUR = 21
  LEN = 32
  COW_REFCNT = 1

  DB<3> Dump( ${"_<a.pl"}[6] );
SV = PVMG(0x55d862786d50) at 0x55d86251fec0
  REFCNT = 1
  FLAGS = (POK,pIOK,pNOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x55d8627d7d50 "use v5.36;\n"\0
  CUR = 11
  LEN = 16

See how "line 8" has IOK and POK, but "line 6" only has POK? That means that line 8 contains an integer in addition to the string, while line 6 contains only the string. This doesn't match the documentation.

(As a side note, why 94387850872920? In hex, it's 55D862528858, which in the same range as the addresses being dumped. So it looks like it's not just a flag, but a pointer to something.)

Note that the problem is limited to lines containing pragmas. The non-pragma unbreakable lines have IOK and a value of zero.

  DB<4> use Devel::Peek;Dump( ${"_<a.pl"}[5] );
SV = PVMG(0x55856a4198d0) at 0x55856a4e9630
  REFCNT = 1
  FLAGS = (IOK,POK,pIOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x55856a486840 "\n"\0
  CUR = 1
  LEN = 16

So only the lines with pragmas are broken. But this wasn't always the case. They used to be broken in a different way. Here with 5.34, we see that pragmas were once breakable.

  DB<2> Dump( ${"_<a.pl"}[6] );
SV = PVMG(0x55874ddaf680) at 0x55874db508a8
  REFCNT = 1
  FLAGS = (IOK,POK,pIOK,pPOK)
  IV = 94039612799016
  NV = 0
  PV = 0x55874db739a0 "use v5.10;\n"\0
  CUR = 11
  LEN = 16

I'm guessing that when they fixed the bug marking pragmas as breakable, they forgot to set the IOK flag.

Bug report

[ikegami]

Fixed in (upcoming) 5.40.

[choroba]

Update: After enabling warnings in the command line, I got the same warnings in 5.39.4. So, disregard this node.

You don't show enough for us to answer. I tried the following in two different versions of Perl (one of them older and one newer than 5.36).

I saved the following script as 2.pl:

#!/usr/bin/perl
use strict;
use feature qw{ say };
use warnings;

my $x = 10;
for my $i (1 .. 10) {
    $x += $i;
}

I then ran

perl -d 2.pl

using both the Perl versions.

5.26.1:

  DB<1> @lines=@{"_<2.pl"} ; for my $i (0 .. $#lines) { CORE::say $lin
+es[$i] == 0 }
1
1



1



1

5.39.4 (blead):

  DB<1> @lines=@{"_<2.pl"} ; for my $i (0 .. $#lines) { CORE::say $lin
+es[$i] == 0 }
1
1
1
1
1
1



1

The output is different, but there's no warning.

[ikegami]

You don't get any warnings because you forgot to enable warnings.

[Anonymous Monk]

I get no warning out of Perl 5.36.1 from the following script:

#!/usr/bin/env perl

use strict;
use warnings;

use v5.36;

say 'Hello, world!';

using the commands:

perl -d dbg.pl
  DB<1> @lines = @{"_<dbg.pl"}; say $_ == 0 for @lines;
1
1
1
1
1
1
1
1


  DB<2> q                                                                       

I also tried 5.36.0, with the same output and the same lack of a warning.

Maybe if you exhibited a small, self-contained example, giving the exact code and exact debugger commands to trigger the problem?

[ikegami]

You don't get any warnings because you forgot to enable warnings.

[Bod]

You don't get any warnings because you forgot to enable warnings.

I see use warnings;

Is this not enabling warnings?