GNU Bison NEWS

* Noteworthy changes in release 3.5.2 (2020-02-13) [stable]

** Bug fixes

  Portability issues and minor cosmetic issues.

  The lalr1.cc skeleton properly rejects unsupported values for parse.lac
  (as yacc.c does).

* Noteworthy changes in release 3.5.1 (2020-01-19) [stable]

** Bug fixes

  Portability fixes.

  Fix compiler warnings.

* Noteworthy changes in release 3.5 (2019-12-11) [stable]

** Backward incompatible changes

  Lone carriage-return characters (aka \r or ^M) in the grammar files are no
  longer treated as end-of-lines.  This changes the diagnostics, and in
  particular their locations.

  In C++, line numbers and columns are now represented as 'int' not
  'unsigned', so that integer overflow on positions is easily checkable via
  'gcc -fsanitize=undefined' and the like.  This affects the API for
  positions.  The default position and location classes now expose
  'counter_type' (int), used to define line and column numbers.

** Deprecated features

  The YYPRINT macro, which works only with yacc.c and only for tokens, was
  obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
  It is deprecated and its support will be removed eventually.

** New features

*** Lookahead correction in C++

  Contributed by Adrian Vogelsgesang.

  The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
  %define variable parse.lac.

*** Variable api.token.raw: Optimized token numbers (all skeletons)

  In the generated parsers, tokens have two numbers: the "external" token
  number as returned by yylex (which starts at 257), and the "internal"
  symbol number (which starts at 3).  Each time yylex is called, a table
  lookup maps the external token number to the internal symbol number.

  When the %define variable api.token.raw is set, tokens are assigned their
  internal number, which saves one table lookup per token, and also saves
  the generation of the mapping table.

  The gain is typically moderate, but in extreme cases (very simple user
  actions), a 10% improvement can be observed.

*** Generated parsers use better types for states

  Stacks now use the best integral type for state numbers, instead of always
  using 15 bits.  As a result "small" parsers now have a smaller memory
  footprint (they use 8 bits), and there is support for large automata (16
  bits), and extra large (using int, i.e., typically 31 bits).

*** Generated parsers prefer signed integer types

  Bison skeletons now prefer signed to unsigned integer types when either
  will do, as the signed types are less error-prone and allow for better
  checking with 'gcc -fsanitize=undefined'.  Also, the types chosen are now
  portable to unusual machines where char, short and int are all the same
  width.  On non-GNU platforms this may entail including <limits.h> and (if
  available) <stdint.h> to define integer types and constants.

*** A skeleton for the D programming language

  For the last few releases, Bison has shipped a stealth experimental
  skeleton: lalr1.d.  It was first contributed by Oliver Mangold, based on
  Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
  H. S. Teoh.

  However, because nobody has committed to improving, testing, and
  documenting this skeleton, it is not clear that it will be supported in
  the future.

  The lalr1.d skeleton *is functional*, and works well, as demonstrated in
  examples/d/calc.d.  Please try it, enjoy it, and... commit to support it.

*** Debug traces in Java

  The Java backend no longer emits code and data for parser tracing if the
  %define variable parse.trace is not defined.

** Diagnostics

*** New diagnostic: -Wdangling-alias

  String literals, which allow for better error messages, are (too)
  liberally accepted by Bison, which might result in silent errors.  For
  instance

    %type <exVal> cond "condition"

  does not define "condition" as a string alias to 'cond' (nonterminal
  symbols do not have string aliases).  It is rather equivalent to

    %nterm <exVal> cond
    %token <exVal> "condition"

  i.e., it gives the type 'exVal' to the "condition" token, which was
  clearly not the intention.

  Also, because string aliases need not be defined, typos such as "baz"
  instead of "bar" will be not reported.

  The option -Wdangling-alias catches these situations.  On

    %token BAR "bar"
    %type <ival> foo "foo"
    %%
    foo: "baz" {}

  bison -Wdangling-alias reports

    warning: string literal not attached to a symbol
          | %type <ival> foo "foo"
          |                  ^~~~~
    warning: string literal not attached to a symbol
          | foo: "baz" {}
          |      ^~~~~

   The -Wall option does not (yet?) include -Wdangling-alias.

*** Better POSIX Yacc compatibility diagnostics

  POSIX Yacc restricts %type to nonterminals.  This is now diagnosed by
  -Wyacc.

    %token TOKEN1
    %type  <ival> TOKEN1 TOKEN2 't'
    %token TOKEN2
    %%
    expr:

  gives with -Wyacc

    input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
        2 | %type  <ival> TOKEN1 TOKEN2 't'
          |               ^~~~~~
    input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
        2 | %type  <ival> TOKEN1 TOKEN2 't'
          |                             ^~~
    input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
        2 | %type  <ival> TOKEN1 TOKEN2 't'
          |                      ^~~~~~

*** Diagnostics with insertion

  The diagnostics now display the suggestion below the underlined source.
  Replacement for undeclared symbols are now also suggested.

    $ cat /tmp/foo.y
    %%
    list: lis '.' |

    $ bison -Wall foo.y
    foo.y:2.7-9: error: symbol 'lis' is used, but is not defined as a token and has no rules; did you mean 'list'?
        2 | list: lis '.' |
          |       ^~~
          |       list
    foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
        2 | list: lis '.' |
          |                ^
          |                %empty
    foo.y: warning: fix-its can be applied.  Rerun with option '--update'. [-Wother]

*** Diagnostics about long lines

  Quoted sources may now be truncated to fit the screen.  For instance, on a
  30-column wide terminal:

    $ cat foo.y
    %token FOO                       FOO                         FOO
    %%
    exp: FOO
    $ bison foo.y
    foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
        1 | …         FOO                  …
          |           ^~~
    foo.y:1.8-10:      previous declaration
        1 | %token FOO                     …
          |        ^~~
    foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
        1 | …         FOO
          |           ^~~
    foo.y:1.8-10:      previous declaration
        1 | %token FOO                     …
          |        ^~~

** Changes

*** Debugging glr.c and glr.cc

  The glr.c skeleton always had asserts to check its own behavior (not the
  user's).  These assertions are now under the control of the parse.assert
  %define variable (disabled by default).

*** Clean up

  Several new compiler warnings in the generated output have been avoided.
  Some unused features are no longer emitted.  Cleaner generated code in
  general.

** Bug Fixes

  Portability issues in the test suite.

  In theory, parsers using %nonassoc could crash when reporting verbose
  error messages. This unlikely bug has been fixed.

  In Java, %define api.prefix was ignored.  It now behaves as expected.

* Noteworthy changes in release 3.4.2 (2019-09-12) [stable]

** Bug fixes

  In some cases, when warnings are disabled, bison could emit tons of white
  spaces as diagnostics.

  When running out of memory, bison could crash (found by fuzzing).

  When defining twice the EOF token, bison would crash.

  New warnings from recent compilers have been addressed in the generated
  parsers (yacc.c, glr.c, glr.cc).

  When lone carriage-return characters appeared in the input file,
  diagnostics could hang forever.

* Noteworthy changes in release 3.4.1 (2019-05-22) [stable]

** Bug fixes

  Portability fixes.

* Noteworthy changes in release 3.4 (2019-05-19) [stable]

** Deprecated features

  The %pure-parser directive is deprecated in favor of '%define api.pure'
  since Bison 2.3b (2008-05-27), but no warning was issued; there is one
  now.  Note that since Bison 2.7 you are strongly encouraged to use
  '%define api.pure full' instead of '%define api.pure'.

** New features

*** Colored diagnostics

  As an experimental feature, diagnostics are now colored, controlled by the
  new options --color and --style.

  To use them, install the libtextstyle library before configuring Bison.
  It is available from

    https://alpha.gnu.org/gnu/gettext/

  for instance

    https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz

  The option --color supports the following arguments:
    - always, yes: Enable colors.
    - never, no: Disable colors.
    - auto, tty (default): Enable colors if the output device is a tty.

  To customize the styles, create a CSS file similar to

    /* bison-bw.css */
    .warning   { }
    .error     { font-weight: 800; text-decoration: underline; }
    .note      { }

  then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
  environment variable to "bison-bw.css".

*** Disabling output

  When given -fsyntax-only, the diagnostics are reported, but no output is
  generated.

  The name of this option is somewhat misleading as bison does more than
  just checking the syntax: every stage is run (including checking for
  conflicts for instance), except the generation of the output files.

*** Include the generated header (yacc.c)

  Before, when --defines is used, bison generated a header, and pasted an
  exact copy of it into the generated parser implementation file.  If the
  header name is not "y.tab.h", it is now #included instead of being
  duplicated.

  To use an '#include' even if the header name is "y.tab.h" (which is what
  happens with --yacc, or when using the Autotools' ylwrap), define
  api.header.include to the exact argument to pass to #include.  For
  instance:

    %define api.header.include {"parse.h"}

  or

    %define api.header.include {<parser/parse.h>}

*** api.location.type is now supported in C (yacc.c, glr.c)

  The %define variable api.location.type defines the name of the type to use
  for locations.  When defined, Bison no longer defines YYLTYPE.

  This can be used in programs with several parsers to factor their
  definition of locations: let one of them generate them, and the others
  just use them.

** Changes

*** Graphviz output

  In conformance with the recommendations of the Graphviz team, if %require
  "3.4" (or better) is specified, the option --graph generates a *.gv file
  by default, instead of *.dot.

*** Diagnostics overhaul

  Column numbers were wrong with multibyte characters, which would also
  result in skewed diagnostics with carets.  Beside, because we were
  indenting the quoted source with a single space, lines with tab characters
  were incorrectly underlined.

  To address these issues, and to be clearer, Bison now issues diagnostics
  as GCC9 does.  For instance it used to display (there's a tab before the
  opening brace):

    foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
     expr: expr '+' "number"        { $$ = $1 + $2; }
                                         ^~
  It now reports

    foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
        3 | expr: expr '+' "number" { $$ = $1 + $2; }
          |                                     ^~

  Other constructs now also have better locations, resulting in more precise
  diagnostics.

*** Fix-it hints for %empty

  Running Bison with -Wempty-rules and --update will remove incorrect %empty
  annotations, and add the missing ones.

*** Generated reports

  The format of the reports (parse.output) was improved for readability.

*** Better support for --no-line.

  When --no-line is used, the generated files are now cleaner: no lines are
  generated instead of empty lines.  Together with using api.header.include,
  that should help people saving the generated files into version control
  systems get smaller diffs.

** Documentation

  A new example in C shows an simple infix calculator with a hand-written
  scanner (examples/c/calc).

  A new example in C shows a reentrant parser (capable of recursive calls)
  built with Flex and Bison (examples/c/reccalc).

  There is a new section about the history of Yaccs and Bison.

** Bug fixes

  A few obscure bugs were fixed, including the second oldest (known) bug in
  Bison: it was there when Bison was entered in the RCS version control
  system, in December 1987.  See the NEWS of Bison 3.3 for the previous
  oldest bug.

* Noteworthy changes in release 3.3.2 (2019-02-03) [stable]

** Bug fixes

  Bison 3.3 failed to generate parsers for grammars with unused nonterminal
  symbols.

* Noteworthy changes in release 3.3.1 (2019-01-27) [stable]

** Changes

  The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
  extensions into errors.  It now makes them simple warnings (-Wyacc).

* Noteworthy changes in release 3.3 (2019-01-26) [stable]

  A new mailing list was created, Bison Announce.  It is low traffic, and is
  only about announcing new releases and important messages (e.g., polls
  about major decisions to make).

  https://lists.gnu.org/mailman/listinfo/bison-announce

** Backward incompatible changes

  Support for DJGPP, which has been unmaintained and untested for years, is
  removed.

** Deprecated features

  A new feature, --update (see below) helps adjusting existing grammars to
  deprecations.

*** Deprecated directives

  The %error-verbose directive is deprecated in favor of '%define
  parse.error verbose' since Bison 3.0, but no warning was issued.

  The '%name-prefix "xx"' directive is deprecated in favor of '%define
  api.prefix {xx}' since Bison 3.0, but no warning was issued.  These
  directives are slightly different, you might need to adjust your code.
  %name-prefix renames only symbols with external linkage, while api.prefix
  also renames types and macros, including YYDEBUG, YYTOKENTYPE,
  yytokentype, YYSTYPE, YYLTYPE, etc.

  Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
  {xx}' will typically have to update YY_DECL from

    #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)

  to

    #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)

*** Deprecated %define variable names

  The following variables, mostly related to parsers in Java, have been
  renamed for consistency.  Backward compatibility is ensured, but upgrading
  is recommended.

    abstract           -> api.parser.abstract
    annotations        -> api.parser.annotations
    extends            -> api.parser.extends
    final              -> api.parser.final
    implements         -> api.parser.implements
    parser_class_name  -> api.parser.class
    public             -> api.parser.public
    strictfp           -> api.parser.strictfp

** New features

*** Generation of fix-its for IDEs/Editors

  When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
  bison now generates machine readable editing instructions to fix some
  issues.  Currently, this is mostly limited to updating deprecated
  directives and removing duplicates.  For instance:

    $ cat foo.y
    %error-verbose
    %define parser_class_name "Parser"
    %define api.parser.class "Parser"
    %%
    exp:;

  See the "fix-it:" lines below:

    $ bison -ffixit foo.y
    foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
     %error-verbose
     ^~~~~~~~~~~~~~
    fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
    foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
     %define parser_class_name "Parser"
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
    foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
     %define api.parser.class "Parser"
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    foo.y:2.1-34:     previous definition
     %define parser_class_name "Parser"
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    fix-it:"foo.y":{3:1-3:34}:""
    foo.y: warning: fix-its can be applied.  Rerun with option '--update'. [-Wother]

  This uses the same output format as GCC and Clang.

*** Updating grammar files

  Fixes can be applied on the fly.  The previous example ends with the
  suggestion to re-run bison with the option -u/--update, which results in a
  cleaner grammar file.

    $ bison --update foo.y
    [...]
    bison: file 'foo.y' was updated (backup: 'foo.y~')

    $ cat foo.y
    %define parse.error verbose
    %define api.parser.class {Parser}
    %%
    exp:;

*** Bison is now relocatable

  If you pass '--enable-relocatable' to 'configure', Bison is relocatable.

  A relocatable program can be moved or copied to a different location on
  the file system.  It can also be used through mount points for network
  sharing.  It is possible to make symbolic links to the installed and moved
  programs, and invoke them through the symbolic link.

*** %expect and %expect-rr modifiers on individual rules

  One can now document (and check) which rules participate in shift/reduce
  and reduce/reduce conflicts.  This is particularly important GLR parsers,
  where conflicts are a normal occurrence.  For example,

      %glr-parser
      %expect 1
      %%

      ...

      argument_list:
        arguments %expect 1
      | arguments ','
      | %empty
      ;

      arguments:
        expression
      | argument_list ',' expression
      ;

      ...

  Looking at the output from -v, one can see that the shift-reduce conflict
  here is due to the fact that the parser does not know whether to reduce
  arguments to argument_list until it sees the token _after_ the following
  ','.  By marking the rule with %expect 1 (because there is a conflict in
  one state), we document the source of the 1 overall shift-reduce conflict.

  In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
  conflicts.  In this case, we mark each of the conflicting rules.  For
  example,

      %glr-parser
      %expect-rr 1

      %%

      stmt:
        target_list '=' expr ';'
      | expr_list ';'
      ;

      target_list:
        target
      | target ',' target_list
      ;

      target:
        ID %expect-rr 1
      ;

      expr_list:
        expr
      | expr ',' expr_list
      ;

      expr:
        ID %expect-rr 1
      | ...
      ;

  In a statement such as

      x, y = 3, 4;

  the parser must reduce x to a target or an expr, but does not know which
  until it sees the '='.  So we notate the two possible reductions to
  indicate that each conflicts in one rule.

  This feature needs user feedback, and might evolve in the future.

*** C++: Actual token constructors

  When variants and token constructors are enabled, in addition to the
  type-safe named token constructors (make_ID, make_INT, etc.), we now
  generate genuine constructors for symbol_type.

  For instance with these declarations

    %token           ':'
       <std::string> ID
       <int>         INT;

  you may use these constructors:

    symbol_type (int token, const std::string&);
    symbol_type (int token, const int&);
    symbol_type (int token);

  Correct matching between token types and value types is checked via
  'assert'; for instance, 'symbol_type (ID, 42)' would abort.  Named
  constructors are preferable, as they offer better type safety (for
  instance 'make_ID (42)' would not even compile), but symbol_type
  constructors may help when token types are discovered at run-time, e.g.,

     [a-z]+   {
                if (auto i = lookup_keyword (yytext))
                  return yy::parser::symbol_type (i);
                else
                  return yy::parser::make_ID (yytext);
              }

*** C++: Variadic emplace

  If your application requires C++11 and you don't use symbol constructors,
  you may now use a variadic emplace for semantic values:

    %define api.value.type variant
    %token <std::pair<int, int>> PAIR

  in your scanner:

    int yylex (parser::semantic_type *lvalp)
    {
      lvalp->emplace <std::pair<int, int>> (1, 2);
      return parser::token::PAIR;
    }

*** C++: Syntax error exceptions in GLR

  The glr.cc skeleton now supports syntax_error exceptions thrown from user
  actions, or from the scanner.

*** More POSIX Yacc compatibility warnings

  More Bison specific directives are now reported with -y or -Wyacc.  This
  change was ready since the release of Bison 3.0 in September 2015.  It was
  delayed because Autoconf used to define YACC as `bison -y`, which resulted
  in numerous warnings for Bison users that use the GNU Build System.

  If you still experience that problem, either redefine YACC as `bison -o
  y.tab.c`, or pass -Wno-yacc to Bison.

*** The tables yyrhs and yyphrs are back

  Because no Bison skeleton uses them, these tables were removed (no longer
  passed to the skeletons, not even computed) in 2008.  However, some users
  have expressed interest in being able to use them in their own skeletons.

** Bug fixes

*** Incorrect number of reduce-reduce conflicts

  On a grammar such as

     exp: "num" | "num" | "num"

  bison used to report a single RR conflict, instead of two.  This is now
  fixed.  This was the oldest (known) bug in Bison: it was there when Bison
  was entered in the RCS version control system, in December 1987.

  Some grammar files might have to adjust their %expect-rr.

*** Parser directives that were not careful enough

  Passing invalid arguments to %nterm, for instance character literals, used
  to result in unclear error messages.

** Documentation

  The examples/ directory (installed in .../share/doc/bison/examples) has
  been restructured per language for clarity.  The examples come with a
  README and a Makefile.  Not only can they be used to toy with Bison, they
  can also be starting points for your own grammars.

  There is now a Java example, and a simple example in C based on Flex and
  Bison (examples/c/lexcalc/).

** Changes

*** Parsers in C++

  They now use noexcept and constexpr.  Please, report missing annotations.

*** Symbol Declarations

  The syntax of the variation directives to declare symbols was overhauled
  for more consistency, and also better POSIX Yacc compliance (which, for
  instance, allows "%type" without actually providing a type).  The %nterm
  directive, supported by Bison since its inception, is now documented and
  officially supported.

  The syntax is now as follows:

    %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
    %left  TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
    %type  TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
    %nterm TAG? ID+ ( TAG ID+ )*

  where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
  such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
  ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
  literal such as ‘"number"’.  The post-fix quantifiers are ‘?’ (zero or
  one), ‘*’ (zero or more) and ‘+’ (one or more).

* Noteworthy changes in release 3.2.4 (2018-12-24) [stable]

** Bug fixes

  Fix the move constructor of symbol_type.

  Always provide a copy constructor for symbol_type, even in modern C++.

* Noteworthy changes in release 3.2.3 (2018-12-18) [stable]

** Bug fixes

  Properly support token constructors in C++ with types that include commas
  (e.g., std::pair<int, int>).  A regression introduced in Bison 3.2.

* Noteworthy changes in release 3.2.2 (2018-11-21) [stable]

** Bug fixes

  C++ portability issues.

* Noteworthy changes in release 3.2.1 (2018-11-09) [stable]

** Bug fixes

  Several portability issues have been fixed in the build system, in the
  test suite, and in the generated parsers in C++.

* Noteworthy changes in release 3.2 (2018-10-29) [stable]

** Backward incompatible changes

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

** Changes

  %printers should use yyo rather than yyoutput to denote the output stream.

  Variant-based symbols in C++ should use emplace() rather than build().

  In C++ parsers, parser::operator() is now a synonym for the parser::parse.

** Documentation

  A new section, "A Simple C++ Example", is a tutorial for parsers in C++.

  A comment in the generated code now emphasizes that users should not
  depend upon non-documented implementation details, such as macros starting
  with YY_.

** 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/c++/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.

* Noteworthy changes in release 3.1 (2018-08-27) [stable]

** Backward incompatible changes

  Compiling Bison now requires a C99 compiler---as announced during the
  release of Bison 3.0, five years ago.  Generated parsers do not require a
  C99 compiler.

  Support for DJGPP, which has been unmaintained and untested for years, is
  obsolete. Unless there is activity to revive it, the next release of Bison
  will have it removed.

** New features

*** Typed midrule actions

  Because their type is unknown to Bison, the values of midrule actions are
  not treated like the others: they don't have %printer and %destructor
  support.  It also prevents C++ (Bison) variants to handle them properly.

  Typed midrule actions address these issues.  Instead of:

    exp: { $<ival>$ = 1; } { $<ival>$ = 2; }   { $$ = $<ival>1 + $<ival>2; }

  write:

    exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; }   { $$ = $1 + $2; }

*** Reports include the type of the symbols

  The sections about terminal and nonterminal symbols of the '*.output' file
  now specify their declared type.  For instance, for:

    %token <ival> NUM

  the report now shows '<ival>':

    Terminals, with rules where they appear

    NUM <ival> (258) 5

*** Diagnostics about useless rules

  In the following grammar, the 'exp' nonterminal is trivially useless.  So,
  of course, its rules are useless too.

    %%
    input: '0' | exp
    exp: exp '+' exp | exp '-' exp | '(' exp ')'

  Previously all the useless rules were reported, including those whose
  left-hand side is the 'exp' nonterminal:

    warning: 1 nonterminal useless in grammar [-Wother]
    warning: 4 rules useless in grammar [-Wother]
    2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
     input: '0' | exp
                  ^^^
    2.14-16: warning: rule useless in grammar [-Wother]
     input: '0' | exp
                  ^^^
    3.6-16: warning: rule useless in grammar [-Wother]
     exp: exp '+' exp | exp '-' exp | '(' exp ')'
          ^^^^^^^^^^^
    3.20-30: warning: rule useless in grammar [-Wother]
     exp: exp '+' exp | exp '-' exp | '(' exp ')'
                        ^^^^^^^^^^^
    3.34-44: warning: rule useless in grammar [-Wother]
     exp: exp '+' exp | exp '-' exp | '(' exp ')'
                                      ^^^^^^^^^^^

  Now, rules whose left-hand side symbol is useless are no longer reported
  as useless.  The locations of the errors have also been adjusted to point
  to the first use of the nonterminal as a left-hand side of a rule:

    warning: 1 nonterminal useless in grammar [-Wother]
    warning: 4 rules useless in grammar [-Wother]
    3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
     exp: exp '+' exp | exp '-' exp | '(' exp ')'
     ^^^
    2.14-16: warning: rule useless in grammar [-Wother]
     input: '0' | exp
                  ^^^

*** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)

  When compiled with exceptions disabled, the generated parsers no longer
  uses try/catch clauses.
