Re: bison-3.1.91 released [beta]

[Akim Demaille]

A problem was found in Bison 3.1.90: in modern C++ the parser stack
failed to grow properly.

So we are very happy to announce Bison 3.1.91, a beta of the forthcoming
Bison 3.2.  Massive improvements were brought to the deterministic C++
skeleton, lalr1.cc.  When variants are enabled and the compiler supports
C++11 or better, move-only types can now be used for semantic values.  C++98
support is not deprecated.

We expect to release Bison 3.2 within a couple of weeks.  If you are a user
of Bison's lalr1.cc, and you are using modern C++ (C++11 or newer), *please*
try this version now.  We are especially interested in usability and
performance feedback: does your grammar look nicer, does your parser run
faster.  Any suggestion will be welcomed.

Many thanks to Frank Heckenbach for paving the way for this release with his
implementation of a C++17 skeleton.


Here are the compressed sources:
  https://alpha.gnu.org/gnu/bison/bison-3.1.91.tar.gz   (4.1MB)
  https://alpha.gnu.org/gnu/bison/bison-3.1.91.tar.xz   (2.1MB)

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This release was bootstrapped with the following tools:
  Autoconf 2.69
  Automake 1.16.1
  Flex 2.6.4
  Gettext 0.19.8.1
  Gnulib v0.1-2176-ga79f2a287

NEWS

* Noteworthy changes in release 3.1.91 (2018-10-18) [beta]

** Bug fixes

  With modern C++, the parser stack failed to grow properly.

* Noteworthy changes in release 3.1.90 (2018-10-17) [beta]

** Backward incompatible changes

  Support for DJGPP, which have been unmaintained and untested for years, is
  obsolete.  Unless there is activity to revive it, it will be removed.

** New features

*** C++: Support for move semantics (lalr1.cc)

  The lalr1.cc skeleton now fully supports C++ move semantics, while
  maintaining compatibility with C++98.  You may now store move-only types
  when using Bison's variants.  For instance:

    %code {
      #include <memory>
      #include <vector>
    }

    %skeleton "lalr1.cc"
    %define api.value.type variant

    %%

    %token <int> INT "int";
    %type <std::unique_ptr<int>> int;
    %type <std::vector<std::unique_ptr<int>>> list;

    list:
      %empty    {}
    | list int  { $$ = std::move($1); $$.emplace_back(std::move($2)); }

    int: "int"  { $$ = std::make_unique<int>($1); }

*** C++: Implicit move of right-hand side values (lalr1.cc)

  In modern C++ (C++11 and later), you should always use 'std::move' with
  the values of the right-hand side symbols ($1, $2, etc.), as they will be
  popped from the stack anyway.  Using 'std::move' is mandatory for
  move-only types such as unique_ptr, and it provides a significant speedup
  for large types such as std::string, or std::vector, etc.

  If '%define api.value.automove' is set, every occurrence '$n' is replaced
  by 'std::move ($n)'.  The second rule in the previous grammar can be
  simplified to:

    list: list int  { $$ = $1; $$.emplace_back($2); }

  With automove enabled, the semantic values are no longer lvalues, so do
  not use the swap idiom:

    list: list int  { std::swap($$, $1); $$.emplace_back($2); }

  This idiom is anyway obsolete: it is preferable to move than to swap.

  A warning is issued when automove is enabled, and a value is used several
  times.

    input.yy:16.31-32: warning: multiple occurrences of $2 with 
api.value.automove enabled [-Wother]
    exp: "twice" exp   { $$ = $2 + $2; }
                                   ^^

  Enabling api.value.automove does not require support for modern C++.  The
  generated code is valid C++98/03, but will use copies instead of moves.

  The new examples/variant-11.yy shows these features in action.

*** C++: The implicit default semantic action is always run

  When variants are enabled, the default action was not run, so

    exp: "number"

  was equivalent to

    exp: "number"  {}

  It now behaves like in all the other cases, as

    exp: "number"  { $$ = $1; }

  possibly using std::move if automove is enabled.

  We do not expect backward compatibility issues.  However, beware of
  forward compatibility issues: if you rely on default actions with
  variants, be sure to '%require "3.2"' to avoid older versions of Bison to
  generate incorrect parsers.

*** C++: Renaming location.hh

  When both %defines and %locations are enabled, Bison generates a
  location.hh file.  If you don't use locations outside of the parser, you
  may avoid its creation with:

    %define api.location.file none

  However this file is useful if, for instance, your parser builds an AST
  decorated with locations: you may use Bison's location independently of
  Bison's parser.  You can now give it another name, for instance:

    %define api.location.file "my-location.hh"

  This name can have directory components, and even be absolute.  The name
  under which the location file is included is controlled by
  api.location.include.

  This way it is possible to have several parsers share the same location
  file.

  For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:

    %locations
    %define api.namespace {foo}
    %define api.location.file "include/ast/loc.hh"
    %define api.location.include {<ast/loc.hh>}

  and use it in src/bar/parser.hh:

    %locations
    %define api.namespace {bar}
    %code requires {#include <ast/loc.hh>}
    %define api.location.type {bar::location}

  Absolute file names are supported, so in your Makefile, passing the flag
  -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
  safe.

*** C++: stack.hh and position.hh are deprecated

  When asked to generate a header file (%defines), the lalr1.cc skeleton
  generates a stack.hh file.  This file had no interest for users; it is now
  made useless: its content is included in the parser definition.  It is
  still generated for backward compatibility.

  When in addition to %defines, location support is requested (%locations),
  the file position.hh is also generated.  It is now also useless: its
  content is now included in location.hh.

  These files are no longer generated when your grammar file requires at
  least Bison 3.2 (%require "3.2").

** Bug fixes

  Portability issues on MinGW and VS2015.

  Portability issues in the test suite.

  Portability/warning issues with Flex.