t/op/caller.t: One test failing once warnings on by default

[jkeenan]

In the t-op-caller-bitmask-problem-20201204 branch, which is forked from alpha-dev-03-warnings, I have created a new file, t/op/xcaller.t, which contains a single unit test extracted from t/op/caller.t.

$ cat t/op/xcaller.t 
#!./perl
# Tests for caller()

#use warnings;
BEGIN {
    chdir 't' if -d 't';
    require './test.pl';
    set_up_inc('../lib');
    plan( tests =>  1 ); # some tests are run in a BEGIN block
}

# The bitmask should be assignable to ${^WARNING_BITS} without resulting in
# different warnings settings.
{
 my $bits = sub { (caller 0)[9] }->();
 my $w;
 local $SIG{__WARN__} = sub { $w++ };
 eval '
   use warnings;
   BEGIN { ${^WARNING_BITS} = $bits }
   local $^W = 1;
   () = 1 + undef;
   $^W = 0;
   () = 1 + undef;
 ';
 is $w, 1, 'value from (caller 0)[9] (bitmask) works in ${^WARNING_BITS}';
}

Once warnings are on by default (as is the case in the branches cited), this test fails.

$ ./perl -Ilib t/op/xcaller.t 
1..1
not ok 1 - value from (caller 0)[9] (bitmask) works in ${^WARNING_BITS}
# Failed test 1 - value from (caller 0)[9] (bitmask) works in ${^WARNING_BITS} at t/op/xcaller.t line 26
#      got "2"
# expected "1"

If I were to put this file in a branch in the Perl 5 core distribution -- where warnings are not on by default -- the test would pass.

$ ./perl -Ilib t/op/xcaller.t 
1..1
ok 1 - value from (caller 0)[9] (bitmask) works in ${^WARNING_BITS}

However, if I were to edit the file there (in Perl 5) to activate the use warnings; statement, the test would fail.

 ./perl -Ilib t/op/ycaller.t 
1..1
not ok 1 - value from (caller 0)[9] (bitmask) works in ${^WARNING_BITS}
# Failed test 1 - value from (caller 0)[9] (bitmask) works in ${^WARNING_BITS} at t/op/ycaller.t line 26
#      got "2"
# expected "1"

So it is clear that turning on warnings by default, causes problems for this test.

@atoomic, can you take a look?

Thank you very much.
Jim Keenan