GNU Bison NEWS

* Noteworthy changes in release 3.6.1 (2020-05-10) [stable]

** Bug fixes

  GNU readline portability issues.

  In C++, yy::parser::symbol_name is now a public member, as was intended.

** New features

  In C++, yy::parser::symbol_type now has a public name() member function.

* Noteworthy changes in release 3.6 (2020-05-08) [stable]

** Backward incompatible changes

  TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".

  The YYERROR_VERBOSE macro is no longer supported; the parsers that still
  depend on it will now produce Yacc-like error messages (just "syntax
  error").  It was superseded by the "%error-verbose" directive in Bison
  1.875 (2003-01-01).  Bison 2.6 (2012-07-19) clearly announced that support
  for YYERROR_VERBOSE would be removed.  Note that since Bison 3.0
  (2013-07-25), "%error-verbose" is deprecated in favor of "%define
  parse.error verbose".

** 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

*** Improved syntax error messages

  Two new values for the %define parse.error variable offer more control to
  the user.  Available in all the skeletons (C, C++, Java).

**** %define parse.error detailed

  The behavior of "%define parse.error detailed" is closely resembling that
  of "%define parse.error verbose" with a few exceptions.  First, it is safe
  to use non-ASCII characters in token aliases (with 'verbose', the result
  depends on the locale with which bison was run).  Second, a yysymbol_name
  function is exposed to the user, instead of the yytnamerr function and the
  yytname table.  Third, token internationalization is supported (see
  below).

**** %define parse.error custom

  With this directive, the user forges and emits the syntax error message
  herself by defining the yyreport_syntax_error function.  A new type,
  yypcontext_t, captures the circumstances of the error, and provides the
  user with functions to get details, such as yypcontext_expected_tokens to
  get the list of expected token kinds.

  A possible implementation of yyreport_syntax_error is:

    int
    yyreport_syntax_error (const yypcontext_t *ctx)
    {
      int res = 0;
      YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
      fprintf (stderr, ": syntax error");
      // Report the tokens expected at this point.
      {
        enum { TOKENMAX = 10 };
        yysymbol_kind_t expected[TOKENMAX];
        int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
        if (n < 0)
          // Forward errors to yyparse.
          res = n;
        else
          for (int i = 0; i < n; ++i)
            fprintf (stderr, "%s %s",
                     i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
      }
      // Report the unexpected token.
      {
        yysymbol_kind_t lookahead = yypcontext_token (ctx);
        if (lookahead != YYSYMBOL_YYEMPTY)
          fprintf (stderr, " before %s", yysymbol_name (lookahead));
      }
      fprintf (stderr, "\n");
      return res;
    }

**** Token aliases internationalization

  When the %define variable parse.error is set to `custom` or `detailed`,
  one may specify which token aliases are to be translated using _().  For
  instance

    %token
        PLUS   "+"
        MINUS  "-"
      <double>
        NUM _("number")
      <symrec*>
        FUN _("function")
        VAR _("variable")

  In that case the user must define _() and N_(), and yysymbol_name returns
  the translated symbol (i.e., it returns '_("variable")' rather that
  '"variable"').  In Java, the user must provide an i18n() function.

*** List of expected tokens (yacc.c)

  Push parsers may invoke yypstate_expected_tokens at any point during
  parsing (including even before submitting the first token) to get the list
  of possible tokens.  This feature can be used to propose autocompletion
  (see below the "bistromathic" example).

  It makes little sense to use this feature without enabling LAC (lookahead
  correction).

*** Returning the error token

  When the scanner returns an invalid token or the undefined token
  (YYUNDEF), the parser generates an error message and enters error
  recovery.  Because of that error message, most scanners that find lexical
  errors generate an error message, and then ignore the invalid input
  without entering the error-recovery.

  The scanners may now return YYerror, the error token, to enter the
  error-recovery mode without triggering an additional error message.  See
  the bistromathic for an example.

*** Deep overhaul of the symbol and token kinds

  To avoid the confusion with types in programming languages, we now refer
  to token and symbol "kinds" instead of token and symbol "types".  The
  documentation and error messages have been revised.

  All the skeletons have been updated to use dedicated enum types rather
  than integral types.  Special symbols are now regular citizens, instead of
  being declared in ad hoc ways.

**** Token kinds

  The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
  LPAREN, etc.  While backward compatibility is of course ensured, users are
  nonetheless invited to replace their uses of "enum yytokentype" by
  "yytoken_kind_t".

  This type now also includes tokens that were previously hidden: YYEOF (end
  of input), YYUNDEF (undefined token), and YYerror (error token).  They
  now have string aliases, internationalized when internationalization is
  enabled.  Therefore, by default, error messages now refer to "end of file"
  (internationalized) rather than the cryptic "$end", or to "invalid token"
  rather than "$undefined".

  Therefore in most cases it is now useless to define the end-of-line token
  as follows:

    %token T_EOF 0 "end of file"

  Rather simply use "YYEOF" in your scanner.

**** Symbol kinds

  The "symbol kinds" is what the parser actually uses.  (Unless the
  api.token.raw %define variable is used, the symbol kind of a terminal
  differs from the corresponding token kind.)

  They are now exposed as a enum, "yysymbol_kind_t".

  This allows users to tailor the error messages the way they want, or to
  process some symbols in a specific way in autocompletion (see the
  bistromathic example below).

*** Modernize display of explanatory statements in diagnostics

  Since Bison 2.7, output was indented four spaces for explanatory
  statements.  For example:

    input.y:2.7-13: error: %type redeclaration for exp
    input.y:1.7-11:     previous declaration

  Since the introduction of caret-diagnostics, it became less clear.  This
  indentation has been removed and submessages are displayed similarly as in
  GCC:

    input.y:2.7-13: error: %type redeclaration for exp
        2 | %type <float> exp
          |       ^~~~~~~
    input.y:1.7-11: note: previous declaration
        1 | %type <int> exp
          |       ^~~~~

  Contributed by Victor Morales Cayuela.

*** C++

  The token and symbol kinds are yy::parser::token_kind_type and
  yy::parser::symbol_kind_type.

  The symbol_type::kind() member function allows to get the kind of a
  symbol.  This can be used to write unit tests for scanners, e.g.,

    yy::parser::symbol_type t = make_NUMBER ("123");
    assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
    assert (t.value.as<int> () == 123);

** Documentation

*** User Manual

  In order to avoid ambiguities with "type" as in "typing", we now refer to
  the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
  type".  We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
  etc.).

*** Examples

  There are now examples/java: a very simple calculator, and a more complete
  one (push-parser, location tracking, and debug traces).

  The lexcalc example (a simple example in C based on Flex and Bison) now
  also demonstrates location tracking.


  A new C example, bistromathic, is a fully featured interactive calculator
  using many Bison features: pure interface, push parser, autocompletion
  based on the current parser state (using yypstate_expected_tokens),
  location tracking, internationalized custom error messages, lookahead
  correction, rich debug traces, etc.

  It shows how to depend on the symbol kinds to tailor autocompletion.  For
  instance it recognizes the symbol kind "VARIABLE" to propose
  autocompletion on the existing variables, rather than of the word
  "variable".

* Noteworthy changes in release 3.5.4 (2020-04-05) [stable]

** WARNING: Future backward-incompatibilities!

  TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".

  Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
  that still depend on it will produce Yacc-like error messages (just
  "syntax error").  It was superseded by the "%error-verbose" directive in
  Bison 1.875 (2003-01-01).  Bison 2.6 (2012-07-19) clearly announced that
  support for YYERROR_VERBOSE would be removed.  Note that since Bison 3.0
  (2013-07-25), "%error-verbose" is deprecated in favor of "%define
  parse.error verbose".

** Bug fixes

  Fix portability issues of the package itself on old compilers.

  Fix api.token.raw support in Java.

* Noteworthy changes in release 3.5.3 (2020-03-08) [stable]

** Bug fixes

  Error messages could quote lines containing zero-width characters (such as
  \005) with incorrect styling.  Fixes for similar issues with unexpectedly
  short lines (e.g., the file was changed between parsing and diagnosing).

  Several unlikely crashes found by fuzzing have been fixed.

* 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
