perldelta for fbc76eb33c6

[perl5.git] / pod / perldelta.pod

=encoding utf8

=head1 NAME

[ this is a template for a new perldelta file.  Any text flagged as XXX needs
to be processed before release. ]

perldelta - what is new for perl v5.21.6

=head1 DESCRIPTION

This document describes differences between the 5.21.5 release and the 5.21.6
release.

If you are upgrading from an earlier release such as 5.21.4, first read
L<perl5215delta>, which describes differences between 5.21.4 and 5.21.5.

=head1 Notice

XXX Any important notices here

=head1 Core Enhancements

XXX New core language features go here.  Summarize user-visible core language
enhancements.  Particularly prominent performance optimisations could go
here, but most should go in the L</Performance Enhancements> section.

[ List each enhancement as a =head2 entry ]

=head2 List form of pipe open implemented for Win32

The list form of pipe:

  open my $fh, "-|", "program", @arguments;

is now implemented on Win32.  It has the same limitations as C<system
LIST> on Win32, since the Win32 API doesn't accept program arguments
as a list.

=head2 Assignment to list repetition

C<(...) x ...> can now be used within a list that is assigned to, as long
as the left-hand side is a valid lvalue.  This allows C<(undef,undef,$foo)
= that_function()> to be written as C<((undef)x2, $foo) = that_function()>.

=head2 C<close> now sets C<$!>

When an I/O error occurs, the fact that there has been an error is recorded
in the handle.  C<close> returns false for such a handle.  Previously, the
value of C<$!> would be untouched by C<close>, so the common convention of
writing C<close $fh or die $!> did not work reliably.  Now the handle
records the value of C<$!>, too, and C<close> restores it.

=head1 Security

XXX Any security-related notices go here.  In particular, any security
vulnerabilities closed should be noted here rather than in the
L</Selected Bug Fixes> section.

[ List each security issue as a =head2 entry ]

=head1 Incompatible Changes

XXX For a release on a stable branch, this section aspires to be:

    There are no changes intentionally incompatible with 5.XXX.XXX
    If any exist, they are bugs, and we request that you submit a
    report.  See L</Reporting Bugs> below.

[ List each incompatible change as a =head2 entry ]

=head1 Deprecations

XXX Any deprecated features, syntax, modules etc. should be listed here.

=head2 Module removals

XXX Remove this section if inapplicable.

The following modules will be removed from the core distribution in a
future release, and will at that time need to be installed from CPAN.
Distributions on CPAN which require these modules will need to list them as
prerequisites.

The core versions of these modules will now issue C<"deprecated">-category
warnings to alert you to this fact.  To silence these deprecation warnings,
install the modules in question from CPAN.

Note that these are (with rare exceptions) fine modules that you are encouraged
to continue to use.  Their disinclusion from core primarily hinges on their
necessity to bootstrapping a fully functional, CPAN-capable Perl installation,
not usually on concerns over their design.

=over

=item XXX

XXX Note that deprecated modules should be listed here even if they are listed
as an updated module in the L</Modules and Pragmata> section.

=back

[ List each other deprecation as a =head2 entry ]

=head2 Use of non-graphic characters in single-character variable names

The syntax for single-character variable names is more lenient than
for longer variable names, allowing the one-character name to be a
punctuation character or even invisible (a non-graphic).  Perl v5.20
deprecated the ASCII-range controls as such a name.  Now, all
non-graphic characters that formerly were allowed are deprecated.
The practical effect of this occurs only when not under C<S<"use
utf8">>, and affects just the C1 controls (code points 0x80 through
0xFF), NO-BREAK SPACE, and SOFT HYPHEN.

=head2 Inlining of C<sub () { $var }> with observable side-effects

In many cases Perl makes sub () { $var } into an inlinable constant
subroutine, capturing the value of $var at the time the C<sub> expression
is evaluated.  This can break the closure behaviour in those cases where
$var is subsequently modified.  The subroutine won't return the new value.

This usage is now deprecated in those cases where the variable could be
modified elsewhere.  Perl detects those cases and emits a deprecation
warning.  Such code will likely change in the future and stop producing a
constant.

If your variable is only modified in the place where it is declared, then
Perl will continue to make the sub inlinable with no warnings.

    sub make_constant {
        my $var = shift;
        return sub () { $var }; # fine
    }

    sub make_constant_deprecated {
        my $var;
        $var = shift;
        return sub () { $bar }; # deprecated
    }

    sub make_constant_deprecated2 {
        my $var = shift;
        log_that_value($var); # could modify $var
        return sub () { $bar }; # deprecated
    }

In the second example above, detecting that $var is assigned to only once
is too hard to detect.  That it happens in a spot other than the C<my>
declaration is enough for Perl to find it suspicious.

This deprecation warning happens only for a simple variable for the body of
the sub.  (A C<BEGIN> block or C<use> statement inside the sub is ignored,
because it does not become part of the sub's body.)  For more complex
cases, such as C<sub () { do_something() if 0; $var }> the behaviour has
changed such that inlining does not happen if the variable is modifiable
elsewhere.  Such cases should be rare.

=head1 Performance Enhancements

XXX Changes which enhance performance without changing behaviour go here.
There may well be none in a stable release.

[ List each enhancement as a =item entry ]

=over 4

=item *

C<(...)x1>, C<("constant")x0> and C<($scalar)x0> are now optimised in list
context.  If the right-hand argument is a constant 1, the repetition
operator disappears.  If the right-hand argument is a constant 0, the whole
expressions is optimised to the empty list, so long as the left-hand
argument is a simple scalar or constant.  C<(foo())x0> is not optimised.

=item *

C<substr> assignment is now optimised into 4-argument C<substr> at the end
of a subroutine (or as the argument to C<return>).  Previously, this
optimisation only happened in void context.

=item *

Assignment to lexical variables is often optimised away.  For instance, in
C<$lexical = chr $foo>, the C<chr> operator writes directly to the lexical
variable instead of returning a value that gets copied.  This optimisation
has been extended to C<split>, C<x> and C<vec> on the right-hand side.  It
has also been made to work with state variable initialization.

=item *

In "\L...", "\Q...", etc., the extra "stringify" op is now optimised away,
making these just as fast as C<lcfirst>, C<quotemeta>, etc.

=item *

Assignment to an empty list is now sometimes faster.  In particular, it
never calls C<FETCH> on tied arguments on the right-hand side, whereas it
used to sometimes.

=back

=head1 Modules and Pragmata

XXX All changes to installed files in F<cpan/>, F<dist/>, F<ext/> and F<lib/>
go here.  If Module::CoreList is updated, generate an initial draft of the
following sections using F<Porting/corelist-perldelta.pl>.  A paragraph summary
for important changes should then be added by hand.  In an ideal world,
dual-life modules would have a F<Changes> file that could be cribbed.

[ Within each section, list entries as a =item entry ]

=head2 New Modules and Pragmata

=over 4

=item *

XXX

=back

=head2 Updated Modules and Pragmata

=over 4

=item *

L<B::Deparse> has been upgraded from version 1.29 to 1.30.

It now deparses C<+sub : attr { ... }> correctly at the start of a
statement.  Without the initial C<+>, C<sub> would be a statement label.

C<BEGIN> blocks are now emitted in the right place most of the time, but
the change unfortunately introduced a regression, in that C<BEGIN> blocks
occurring just before the end of the enclosing block may appear below it
instead.  So this change may need to be reverted if it cannot be fixed
before Perl 5.22.  [perl #77452]

B::Deparse no longer puts erroneous C<local> here and there, such as for
C<LIST = tr/a//d>.  [perl #119815]

=item *

L<B::Op_private> has been upgraded from version 5.021005 to 5.021006.

It now includes a hash named C<%ops_using>, list all op types that use a
particular private flag.

=item *

L<DynaLoader> has been upgraded from version 1.27 to 1.28.

=item *

L<IO::Socket> has been upgraded from version 1.37 to 1.38.

Document the limitations of the isconnected() method.  [perl #123096]

=back

=head2 Removed Modules and Pragmata

=over 4

=item *

XXX

=back

=head1 Documentation

XXX Changes to files in F<pod/> go here.  Consider grouping entries by
file and be sure to link to the appropriate page, e.g. L<perlfunc>.

=head2 New Documentation

XXX Changes which create B<new> files in F<pod/> go here.

=head3 L<XXX>

XXX Description of the purpose of the new file here

=head2 Changes to Existing Documentation

XXX Changes which significantly change existing files in F<pod/> go here.
However, any changes to F<pod/perldiag.pod> should go in the L</Diagnostics>
section.

=head3 L<perldata/Identifier parsing>

=over 4

=item *

The syntax of single-character variable names has been brought
up-to-date and more fully explained.

=back

=head1 Diagnostics

The following additions or changes have been made to diagnostic output,
including warnings and fatal error messages.  For the complete list of
diagnostic messages, see L<perldiag>.

XXX New or changed warnings emitted by the core's C<C> code go here.  Also
include any changes in L<perldiag> that reconcile it to the C<C> code.

=head2 New Diagnostics

XXX Newly added diagnostic messages go under here, separated into New Errors
and New Warnings

=head3 New Errors

=over 4

=item *

XXX L<message|perldiag/"message">

=back

=head3 New Warnings

=over 4

=item *

L<Use of literal non-graphic characters in variable names is deprecated|perldiag/"Use of literal non-graphic characters in variable names is deprecated">

=item *

A new C<locale> warning category has been created, with the following warning
messages currently in it:

=over 4

=item *

L<Locale '%s' may not work well.%s|perldiag/Locale '%s' may not work well.%s>

=item *

L<Can't do %s("%s") on non-UTF-8 locale; resolved to "%s".|perldiag/Can't do %s("%s") on non-UTF-8 locale; resolved to "%s".>

=back

=item *

L<Warning: unable to close filehandle %s properly: %s|perldiag/"Warning: unable to close filehandle %s properly: %s">

=item *

The following two warnings for C<tr///> used to be skipped if the
transliteration contained wide characters, but now they occur regardless of
whether there are wide characters or not:

L<Useless use of E<sol>d modifier in transliteration operator|perldiag/"Useless use of /d modifier in transliteration operator">

L<Replacement list is longer than search list|perldiag/Replacement list is longer than search list>

=back

=head2 Changes to Existing Diagnostics

XXX Changes (i.e. rewording) of diagnostic messages go here

=over 4

=item *

L<Quantifier unexpected on zero-length expression in regex mE<sol>%sE<sol>|perldiag/"Quantifier unexpected on zero-length expression in regex m/%s/">.

This message has had the S<"<-- HERE"> marker removed, as it was always
placed at the end of the regular expression, regardless of where the
problem actually occurred.  [perl #122680]

=back

=head1 Utility Changes

XXX Changes to installed programs such as F<perlbug> and F<xsubpp> go here.
Most of these are built within the directory F<utils>.

[ List utility changes as a =head2 entry for each utility and =item
entries for each change
Use L<XXX> with program names to get proper documentation linking. ]

=head2 L<XXX>

=over 4

=item *

XXX

=back

=head1 Configuration and Compilation

XXX Changes to F<Configure>, F<installperl>, F<installman>, and analogous tools
go here.  Any other changes to the Perl build process should be listed here.
However, any platform-specific changes should be listed in the
L</Platform Support> section, instead.

[ List changes as a =item entry ].

=over 4

=item *

F<Configure> with C<-Dmksymlinks> should now be faster. [perl #122002]

=back

=head1 Testing

XXX Any significant changes to the testing of a freshly built perl should be
listed here.  Changes which create B<new> files in F<t/> go here as do any
large changes to the testing harness (e.g. when parallel testing was added).
Changes to existing files in F<t/> aren't worth summarizing, although the bugs
that they represent may be covered elsewhere.

[ List each test improvement as a =item entry ]

=over 4

=item *

XXX

=back

=head1 Platform Support

XXX Any changes to platform support should be listed in the sections below.

[ Within the sections, list each platform as a =item entry with specific
changes as paragraphs below it. ]

=head2 New Platforms

XXX List any platforms that this version of perl compiles on, that previous
versions did not.  These will either be enabled by new files in the F<hints/>
directories, or new subdirectories and F<README> files at the top level of the
source tree.

=over 4

=item XXX-some-platform

XXX

=back

=head2 Discontinued Platforms

XXX List any platforms that this version of perl no longer compiles on.

=over 4

=item XXX-some-platform

XXX

=back

=head2 Platform-Specific Notes

XXX List any changes for specific platforms.  This could include configuration
and compilation changes or changes in portability/compatibility.  However,
changes within modules for platforms should generally be listed in the
L</Modules and Pragmata> section.

=over 4

=item XXX-some-platform

XXX

=back

=head3 Win32

=over 4

=item *

In the experimental C<:win32> layer, a crash in C<open> was fixed. Also
opening C</dev/null>, which works the Win32 Perl's normal C<:unix> layer, was
implemented for C<:win32>.
L<[perl #122224]|https://rt.perl.org/Ticket/Display.html?id=122224>

=item *

A new makefile option, C<USE_LONG_DOUBLE>, has been added to the Windows
dmake makefile for gcc builds only.  Set this to "define" if you want perl to
use long doubles to give more accuracy and range for floating point numbers.

=back

=head1 Internal Changes

XXX Changes which affect the interface available to C<XS> code go here.  Other
significant internal changes for future core maintainers should be noted as
well.

[ List each change as a =item entry ]

=over 4

=item *

C<screaminstr> has been removed. Although marked as public API, it is
undocumented and has no usage in modern perl versions on CPAN Grep. Calling it
has been fatal since 5.17.0.

=item *

C<newDEFSVOP>, C<block_start>, C<block_end> and C<intro_my> have been added
to the API.

=item *

The internal C<convert> function in F<op.c> has been renamed
C<op_convert_list> and added to the API.

=item *

C<sv_magic> no longer forbids "ext" magic on read-only values.  After all,
perl can't know whether the custom magic will modify the SV or not.
[perl #123103]

=back

=head1 Selected Bug Fixes

XXX Important bug fixes in the core language are summarized here.  Bug fixes in
files in F<ext/> and F<lib/> are best summarized in L</Modules and Pragmata>.

[ List each fix as a =item entry ]

=over 4

=item *

fchmod() and futimes() now set C<$!> when they fail due to being
passed a closed file handle.  [perl #122703]

=item *

Perl now comes with a corrected Unicode 7.0 for the erratum issued on
October 21, 2014 (see L<http://www.unicode.org/errata/#current_errata>),
dealing with glyph shaping in Arabic.

=item *

op_free() no longer crashes due to a stack overflow when freeing a
deeply recursive op tree. [perl #108276]

=item *

scalarvoid() would crash due to a stack overflow when processing a
deeply recursive op tree. [perl #108276]

=item *

In Perl 5.20.0, C<$^N> accidentally had the internal UTF8 flag turned off
if accessed from a code block within a regular expression, effectively
UTF8-encoding the value.  This has been fixed.  [perl #123135]

=item *

A failed C<semctl> call no longer overwrites existing items on the stack,
causing C<(semctl(-1,0,0,0))[0]> to give an "uninitialized" warning.

=item *

C<else{foo()}> with no space before C<foo> is now better at assigning the
right line number to that statement.  [perl #122695]

=item *

Sometimes the assignment in C<@array = split> gets optimised and C<split>
itself writes directly to the array.  This caused a bug, preventing this
assignment from being used in lvalue context.  So
C<(@a=split//,"foo")=bar()> was an error.  (This bug probably goes back to
Perl 3, when the optimisation was added.)  This optimisation, and the bug,
started to happen in more cases in 5.21.5.  It has now been fixed.
[perl #123057]

=item *

When argument lists that fail the checks installed by subroutine
signatures, the resulting error messages now give the file and line number
of the caller, not of the called subroutine.  [perl #121374]

=item *

Flip-flop operators (C<..> and C<...> in scalar context) used to maintain
a separate state for each recursion level (the number of times the
enclosing sub was called recursively), contrary to the documentation.  Now
each closure has one internal state for each flip-flop.  [perl #122829]

=item *

C<use>, C<no>, statement labels, special blocks (C<BEGIN>) and pod are now
permitted as the first thing in a C<map> or C<grep> block, the block after
C<print> or C<say> (or other functions) returning a handle, and within
C<${...}>, C<@{...}>, etc.  [perl #122782]

=item *

The repetition operator C<x> now propagates lvalue context to its left-hand
argument when used in contexts like C<foreach>.  That allows
C<for(($#that_array)x2) { ... }> to work as expected if the loop modifies
$_.

=item *

C<(...) x ...> in scalar context used to corrupt the stack if one operand
were an object with "x" overloading, causing erratic behaviour.
[perl #121827]

=item *

Assignment to a lexical scalar is often optimised away (as mentioned under
L</Performance Enhancements>).  Various bugs related to this optimisation
have been fixed.  Certain operators on the right-hand side would sometimes
fail to assign the value at all or assign the wrong value, or would call
STORE twice or not at all on tied variables.  The operators affected were
C<$foo++>, C<$foo-->, and C<-$foo> under C<use integer>, C<chomp>, C<chr>
and C<setpgrp>.

=item *

List assignments were sometimes buggy if the same scalar ended up on both
sides of the assignment due to used of C<tied>, C<values> or C<each>.  The
result would be the wrong value getting assigned.

=item *

C<setpgrp($nonzero)> (with one argument) was accidentally changed in 5.16
to mean C<setpgrp(0)>.  This has been fixed.

=item *

C<__SUB__> could return the wrong value or even corrupt memory under the
debugger (the B<-d> switch) and in subs containing C<eval $string>.

=item *

When C<sub () { $var }> becomes inlinable, it now returns a different
scalar each time, just as a non-inlinable sub would, though Perl still
optimises the copy away in cases where it would make no observable
difference.

=item *

C<my sub f () { $var }> and C<sub () : attr { $var }> are no longer
eligible for inlining.  The former would crash; the latter would just
throw the attributes away.  An exception is made for the little-known
":method" attribute, which does nothing much.

=item *

Inlining of subs with an empty prototype is now more consistent than
before.  Previously, a sub with multiple statements, all but the last
optimised away, would be inlinable only if it were an anonymous sub
containing a string C<eval> or C<state> declaration or closing over an
outer lexical variable (or any anonymous sub under the debugger).  Now any
sub that gets folded to a single constant after statements have been
optimised away is eligible for inlining.  This applies to things like C<sub
() { jabber() if DEBUG; 42 }>.

Some subroutines with an explicit C<return> were being made inlinable,
contrary to the documentation,  Now C<return> always prevents inlining.

=item *

On some systems, such as VMS, C<crypt> can return a non-ASCII string.  If a
scalar assigned to had contained a UTF8 string previously, then C<crypt>
would not turn off the UTF8 flag, thus corrupting the return value.  This
would happen with C<$lexical = crypt ...>. 

=back

=head1 Known Problems

XXX Descriptions of platform agnostic bugs we know we can't fix go here.  Any
tests that had to be C<TODO>ed for the release would be noted here.  Unfixed
platform specific bugs also go here.

[ List each fix as a =item entry ]

=over 4

=item *

Starting in 5.21.6, accessing L<perlapi/CvPADLIST> in an XSUB is forbidden.
CvPADLIST has be reused for a different internal purpose for XSUBs. Guard all
CvPADLIST expressions with C<CvISXSUB()> if your code doesn't already block
XSUB CV*s from going through optree CV* expecting code.

=back

=head1 Errata From Previous Releases

=over 4

=item *

Due to a mistake in the string-copying logic, copying the value of a state
variable could instead steal the value and undefine the variable.  This
bug, introduced in 5.20, would happen mostly for long strings (1250 chars
or more), but could happen for any strings under builds with copy-on-write
disabled.  [perl #123029]

This bug was actually fixed in 5.21.5, but it was not until after that
release that this bug, and the fact that it had been fixed, were
discovered.

=item *

If a named sub tries to access a scalar declared in an outer anonymous sub,
the variable is not available, so the named sub gets its own undefined
scalar.  In 5.10, attempts to take a reference to the variable
(C<\$that_variable>) began returning a reference to a I<copy> of it
instead.  This was accidentally fixed in 5.21.4, but the bug and its fix
were not noticed till now.

=back

=head1 Obituary

XXX If any significant core contributor has died, we've added a short obituary
here.

=head1 Acknowledgements

XXX Generate this with:

  perl Porting/acknowledgements.pl v5.21.5..HEAD

=head1 Reporting Bugs

If you find what you think is a bug, you might check the articles recently
posted to the comp.lang.perl.misc newsgroup and the perl bug database at
https://rt.perl.org/ .  There may also be information at
http://www.perl.org/ , the Perl Home Page.

If you believe you have an unreported bug, please run the L<perlbug> program
included with your release.  Be sure to trim your bug down to a tiny but
sufficient test case.  Your bug report, along with the output of C<perl -V>,
will be sent off to perlbug@perl.org to be analysed by the Perl porting team.

If the bug you are reporting has security implications, which make it
inappropriate to send to a publicly archived mailing list, then please send it
to perl5-security-report@perl.org.  This points to a closed subscription
unarchived mailing list, which includes all the core committers, who will be
able to help assess the impact of issues, figure out a resolution, and help
co-ordinate the release of patches to mitigate or fix the problem across all
platforms on which Perl is supported.  Please only use this address for
security issues in the Perl core, not for modules independently distributed on
CPAN.

=head1 SEE ALSO

The F<Changes> file for an explanation of how to view exhaustive details on
what changed.

The F<INSTALL> file for how to build Perl.

The F<README> file for general stuff.

The F<Artistic> and F<Copying> files for copyright information.

=cut