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Warn when refalias desynchronises a closure-captured variable #24026

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Warn when refalias desynchronises a closure-captured variable #24026

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969375f
Add comment in perl.h about PNf/PNfARG for printing PADNAME
leonerd Dec 16, 2025
260c8d6
Warn when `refalias` desynchronises a closure-captured variable
leonerd Dec 22, 2025
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3 changes: 3 additions & 0 deletions perl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -4107,6 +4107,9 @@ out there, Solaris being the most prominent.
#endif
#define UTF8fARG(u,l,p) (int)cBOOL(u), (UV)(l), (void*)(p)

/* For printing a PADNAME. Ideally these would have been named
* PADNAMEf and PADNAMEfARG
*/
#define PNf UTF8f
#define PNfARG(pn) (int)1, (UV)PadnameLEN(pn), (void *)PadnamePV(pn)

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9 changes: 9 additions & 0 deletions pod/perldiag.pod
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Expand Up @@ -8328,6 +8328,15 @@ anonymous, using the C<sub {}> syntax. When inner anonymous subs that
reference variables in outer subroutines are created, they
are automatically rebound to the current values of such variables.

=item Variable "%s" will not stay shared after refalias

(W closure) A refalias operation has been performed on a variable that is
captured by one or more closures, and thus is shared between them. As a
result of the way it is currently implemented, the variable in the scope
that performed has now become disconnected from those in other scopes, and
now operates independently. Updates to this variable will not be reflected
in the others, or vice versa.

=item vector argument not supported with alpha versions

(S printf) The %vd (s)printf format does not support version objects
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6 changes: 6 additions & 0 deletions pp.c
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Expand Up @@ -7624,6 +7624,12 @@ PP(pp_refassign)
case 0:
{
SV * const old = PAD_SV(ARGTARG);
if(SvREFCNT(old) > 1) {
PADNAME *pn = PadlistNAMESARRAY(CvPADLIST(find_runcv(NULL)))[ARGTARG];
warner(packWARN(WARN_CLOSURE),
"Variable \"%" PNf "\" will not stay shared after refalias",
PNfARG(pn));
}
PAD_SETSV(ARGTARG, SvREFCNT_inc_NN(SvRV(sv)));
SvREFCNT_dec(old);
if ((PL_op->op_private & (OPpLVAL_INTRO|OPpPAD_STATE))
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9 changes: 8 additions & 1 deletion t/op/lvref.t
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Expand Up @@ -5,7 +5,7 @@ BEGIN {
set_up_inc("../lib");
}

plan 203;
plan 204;

eval '\$x = \$y';
like $@, qr/^Experimental aliasing via reference not enabled/,
Expand Down Expand Up @@ -626,6 +626,9 @@ like $@,
# Miscellaneous

{
my $warnings = "";
local $SIG{__WARN__} = sub { $warnings .= $_[0]; };

local $::TODO = ' ';
my($x,$y);
sub {
Expand All @@ -635,6 +638,10 @@ like $@,
is \$x, \$y, 'lexical alias affects outer closure';
}->();
is \$x, \$y, 'lexical alias affects outer sub where vars are declared';

undef $::TODO;
like $warnings, qr/^Variable "\$x" will not stay shared after refalias /,
'warning emitted by refalias on closure shared variable';
}

{ # PADSTALE has a double meaning
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