+** WARNING: Future backward-incompatibilities!
+
+ Bison will stop adding a semicolon at the end of the actions (as announced
+ in the release 2.5):
+
+ foo.y:2.22: warning: a ';' might be needed at the end of action code
+ exp: "num" { $$ = $1 }
+ ^
+ foo.y:2.22: future versions of Bison will not add the ';'
+
+ Like other GNU packages, Bison will start using some of the C99 features
+ for its own code, especially the definition of variables after statements.
+ The generated C parsers still aim at C90.
+
+** Backward incompatible changes
+
+*** Obsolete features
+
+ Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
+
+ Support for yystype and yyltype is removed (deprecated in Bison 1.875):
+ use YYSTYPE and YYLTYPE.
+
+ Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
+ 1.875): use %lex-param, %parse-param, or %param.
+
+** Bug fixes
+
+*** The epilogue is no longer affected by internal #defines
+
+ The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
+ generated code. These weren't properly undefined before the inclusion of
+ the user epilogue, so functions such as the following were butchered by the
+ preprocessor expansion:
+
+ int yylex (YYSTYPE *yylval);
+
+ This is has been fixed: yylval, yynerrs, yychar, and yylloc are now valid
+ identifiers for user-provided variables.
+
+*** stdio.h is no longer needed when locations are enabled (yacc.c)
+
+ Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
+ locations are enabled. This is fixed.
+
+** Diagnostics reported by Bison
+
+ Most of these features were contributed by Théophile Ranquet and Victor
+ Santet.
+
+*** Carets
+
+ Version 2.7 introduced caret errors, for a prettier output. These are now
+ activated by default. The old format can still be used by invoking Bison
+ with -fno-caret (or -fnone).
+
+*** Enhancements of the -Werror option
+
+ The -Werror=CATEGORY option is now recognized, and will treat specified
+ warnings as errors. The warnings need not have been explicitly activated
+ using the -W option, this is similar to what GCC 4.7 does.
+
+ For example, given the following command line, Bison will treat both
+ warnings related to POSIX Yacc incompatibilities and S/R conflicts as
+ errors (and only those):
+
+ $ bison -Werror=yacc,error=conflicts-sr input.y
+
+ If no categories are specified, -Werror will make all active warnings into
+ errors. For example, the following line does the same the previous example:
+
+ $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
+
+ (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
+
+ Note that the categories in this -Werror option may not be prefixed with
+ "no-". However, -Wno-error[=CATEGORY] is valid.
+
+ Note that -y enables -Werror=yacc. Therefore it is now possible to require
+ Yacc-like behavior (e.g., always generate y.tab.c), but to report
+ incompatibilities as warnings: "-y -Wno-error=yacc".
+
+*** The display of warnings is now richer
+
+ The option that controls a given warning is now displayed:
+
+ foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
+
+ In the case of warnings treated as errors, the prefix is changed from
+ "warning: " to "error: ", and the suffix is displayed, in a manner similar
+ to GCC, as [-Werror=CATEGORY].
+
+ For instance, where the previous version of Bison would report (and exit
+ with failure):
+
+ bison: warnings being treated as errors
+ input.y:1.1: warning: stray ',' treated as white space
+
+ it now reports:
+
+ input.y:1.1: error: stray ',' treated as white space [-Werror=other]
+
+*** Deprecated constructs
+
+ The new 'deprecated' warning category flags obsolete constructs whose
+ support will be discontinued. It is enabled by default. These warnings
+ used to be reported as 'other' warnings.
+
+*** Useless semantic types
+
+ Bison now warns about useless (uninhabited) semantic types. Since
+ semantic types are not declared to Bison (they are defined in the opaque
+ %union structure), it is %printer/%destructor directives about useless
+ types that trigger the warning:
+
+ %token <type1> term
+ %type <type2> nterm
+ %printer {} <type1> <type3>
+ %destructor {} <type2> <type4>
+ %%
+ nterm: term { $$ = $1; };
+
+ 3.28-34: warning: type <type3> is used, but is not associated to any symbol
+ 4.28-34: warning: type <type4> is used, but is not associated to any symbol
+
+*** Undefined but unused symbols
+
+ Bison used to raise an error for undefined symbols that are not used in
+ the grammar. This is now only a warning.
+
+ %printer {} symbol1
+ %destructor {} symbol2
+ %type <type> symbol3
+ %%
+ exp: "a";
+
+*** Useless destructors or printers
+
+ Bison now warns about useless destructors or printers. In the following
+ example, the printer for <type1>, and the destructor for <type2> are
+ useless: all symbols of <type1> (token1) already have a printer, and all
+ symbols of type <type2> (token2) already have a destructor.
+
+ %token <type1> token1
+ <type2> token2
+ <type3> token3
+ <type4> token4
+ %printer {} token1 <type1> <type3>
+ %destructor {} token2 <type2> <type4>
+
+*** Conflicts
+
+ The warnings and error messages about shift/reduce and reduce/reduce
+ conflicts have been normalized. For instance on the following foo.y file:
+
+ %glr-parser
+ %%
+ exp: exp '+' exp | '0' | '0';
+
+ compare the previous version of bison:
+
+ $ bison foo.y
+ foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
+ $ bison -Werror foo.y
+ bison: warnings being treated as errors
+ foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
+
+ with the new behavior:
+
+ $ bison foo.y
+ foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+ foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
+ $ bison -Werror foo.y
+ foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
+ foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
+
+ When %expect or %expect-rr is used, such as with bar.y:
+
+ %expect 0
+ %glr-parser
+ %%
+ exp: exp '+' exp | '0' | '0';
+
+ Former behavior:
+
+ $ bison bar.y
+ bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
+ bar.y: expected 0 shift/reduce conflicts
+ bar.y: expected 0 reduce/reduce conflicts
+
+ New one:
+
+ $ bison bar.y
+ bar.y: error: shift/reduce conflicts: 1 found, 0 expected
+ bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
+
+** Additional yylex/yyparse arguments
+
+ The new directive %param declares additional arguments to both yylex and
+ yyparse. The %lex-param, %parse-param, and %param directives support one
+ or more arguments. Instead of
+
+ %lex-param {arg1_type *arg1}
+ %lex-param {arg2_type *arg2}
+ %parse-param {arg1_type *arg1}
+ %parse-param {arg2_type *arg2}
+
+ one may now declare
+
+ %param {arg1_type *arg1} {arg2_type *arg2}
+
+** Java skeleton improvements
+
+ Contributed by Paolo Bonzini.
+
+ The constants for token names were moved to the Lexer interface. Also, it
+ is possible to add code to the parser's constructors using "%code init"
+ and "%define init_throws".
+
+** C++ skeletons improvements
+
+*** The parser header is no longer mandatory (lalr1.cc, glr.cc)
+
+ Using %defines is now optional. Without it, the needed support classes
+ are defined in the generated parser, instead of additional files (such as
+ location.hh, position.hh and stack.hh).
+
+*** Locations are no longer mandatory (lalr1.cc, glr.cc)
+
+ Both lalr1.cc and glr.cc no longer require %location.
+
+*** syntax_error exception (lalr1.cc)
+
+ The C++ parser features a syntax_error exception, which can be
+ thrown from the scanner or from user rules to raise syntax errors.
+ This facilitates reporting errors caught in sub-functions (e.g.,
+ rejecting too large integral literals from a conversion function
+ used by the scanner, or rejecting invalid combinations from a
+ factory invoked by the user actions).
+
+** Renamed %define variables
+
+ The following variables have been renamed for consistency. Backward
+ compatibility is ensured, but upgrading is recommended.
+
+ lr.default-reductions -> lr.default-reduction
+ lr.keep-unreachable-states -> lr.keep-unreachable-state
+ namespace -> api.namespace
+ stype -> api.value.type
+
+** Variable api.token.prefix
+
+ The variable api.token.prefix changes the way tokens are identified in
+ the generated files. This is especially useful to avoid collisions
+ with identifiers in the target language. For instance
+
+ %token FILE for ERROR
+ %define api.token.prefix "TOK_"
+ %%
+ start: FILE for ERROR;
+
+ will generate the definition of the symbols TOK_FILE, TOK_for, and
+ TOK_ERROR in the generated sources. In particular, the scanner must
+ use these prefixed token names, although the grammar itself still
+ uses the short names (as in the sample rule given above).
+
+** Variable parse.error
+
+ This variable controls the verbosity of error messages. The use of the
+ %error-verbose directive is deprecated in favor of "%define parse.error
+ verbose".
+
+** Semantic predicates
+
+ Contributed by Paul Hilfinger.
+
+ The new, experimental, semantic-predicate feature allows actions of the
+ form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
+ YYERROR) if the expression evaluates to 0, and are evaluated immediately
+ in GLR parsers, rather than being deferred. The result is that they allow
+ the programmer to prune possible parses based on the values of run-time
+ expressions.
+
+** The directive %expect-rr is now an error in non GLR mode
+
+ It used to be an error only if used in non GLR mode, _and_ if there are
+ reduce/reduce conflicts.
+
+** Token numbering has changed to preserve the user-defined order
+
+ When declaring %token A B, the numbering for A is inferior to B. Up to now,
+ when declaring associativity at the same time, with %left (or %right,
+ %precedence, %nonassoc), B was inferior to A.
+
+** Useless precedence and associativity
+
+ Contributed by Valentin Tolmer.
+
+ When developping and maintaining a grammar, useless associativity and
+ precedence directives are common. They can be a nuisance: new ambiguities
+ arising are sometimes masked because their conflicts are resolved due to
+ the extra precedence or associativity information. Furthermore, it can
+ hinder the comprehension of a new grammar: one will wonder about the role
+ of a precedence, where in fact it is useless. The following changes aim
+ at detecting and reporting these extra directives.
+
+*** Precedence warning category
+
+ A new category of warning, -Wprecedence, was introduced. It flags the
+ useless precedence and associativity directives.
+
+*** Useless associativity
+
+ Bison now warns about symbols with a declared associativity that is never
+ used to resolve conflicts. In that case, using %precedence is sufficient;
+ the parsing tables will remain unchanged. Solving these warnings may raise
+ useless precedence warnings, as the symbols no longer have associativity.
+ For example:
+
+ %left '+'
+ %left '*'
+ %%
+ exp:
+ "num"
+ | exp '+' "num"
+ | exp '*' exp
+ ;
+
+ will produce a
+
+ warning: useless associativity for '+', use %precedence [-Wprecedence]
+ %left '+'
+ ^^^
+
+*** Useless precedence
+
+ Bison now warns about symbols with a declared precedence and no declared
+ associativity (i.e., declared with %precedence), and whose precedence is
+ never used. In that case, the symbol can be safely declared with %token
+ instead, without modifying the parsing tables. For example:
+
+ %precedence '='
+ %%
+ exp: "var" '=' "num";
+
+ will produce a
+
+ warning: useless precedence for '=' [-Wprecedence]
+ %precedence '='
+ ^^^
+
+*** Useless precedence and associativity
+
+ In case of both useless precedence and associativity, the issue is flagged
+ as follows:
+
+ %nonassoc '='
+ %%
+ exp: "var" '=' "num";
+
+ The warning is:
+
+ warning: useless precedence and associativity for '=' [-Wprecedence]
+ %nonassoc '='
+ ^^^
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