Implement __VA_OPT__

This implements __VA_OPT__, a new preprocessor feature added in C++2A.
The paper can be found here:

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0306r4.html

gcc/ChangeLog

        * doc/cpp.texi (Variadic Macros): Document __VA_OPT__.

gcc/testsuite/ChangeLog

        * c-c++-common/cpp/va-opt-pedantic.c: New file.
        * c-c++-common/cpp/va-opt.c: New file.
        * c-c++-common/cpp/va-opt-error.c: New file.

libcpp/ChangeLog

        * pch.c (cpp_read_state): Set n__VA_OPT__.
        * macro.c (vaopt_state): New class.
        (_cpp_arguments_ok): Check va_opt flag.
        (replace_args, create_iso_definition): Use vaopt_state.
        * lex.c (lex_identifier_intern): Possibly issue errors for
        __VA_OPT__.
        (lex_identifier): Likewise.
        (maybe_va_opt_error): New function.
        * internal.h (struct lexer_state) <va_args_ok>: Update comment.
        (struct spec_nodes) <n__VA_OPT__>: New field.
        * init.c (struct lang_flags) <va_opt>: New field.
        (lang_defaults): Add entries for C++2A.  Update all entries for
        va_opt.
        (cpp_set_lang): Initialize va_opt.
        * include/cpplib.h (struct cpp_options) <va_opt>: New field.
        * identifiers.c (_cpp_init_hashtable): Initialize n__VA_OPT__.

From-SVN: r254707

1 files changed, 36 insertions, 10 deletions

diff --git a/gcc/doc/cpp.texi b/gcc/doc/cpp.texi
index 8cafb65..94437d5 100644
--- a/gcc/doc/cpp.texi
+++ b/gcc/doc/cpp.texi
@@ -1675,20 +1675,27 @@ macro.  We could define @code{eprintf} like this, instead:
 @end smallexample
 
 @noindent
-This formulation looks more descriptive, but unfortunately it is less
-flexible: you must now supply at least one argument after the format
-string.  In standard C, you cannot omit the comma separating the named
-argument from the variable arguments.  Furthermore, if you leave the
-variable argument empty, you will get a syntax error, because
-there will be an extra comma after the format string.
+This formulation looks more descriptive, but historically it was less
+flexible: you had to supply at least one argument after the format
+string.  In standard C, you could not omit the comma separating the
+named argument from the variable arguments.  (Note that this
+restriction has been lifted in C++2a, and never existed in GNU C; see
+below.)
+
+Furthermore, if you left the variable argument empty, you would have
+gotten a syntax error, because there would have been an extra comma
+after the format string.
 
 @smallexample
 eprintf("success!\n", );
      @expansion{} fprintf(stderr, "success!\n", );
 @end smallexample
 
-GNU CPP has a pair of extensions which deal with this problem.  First,
-you are allowed to leave the variable argument out entirely:
+This has been fixed in C++2a, and GNU CPP also has a pair of
+extensions which deal with this problem.
+
+First, in GNU CPP, and in C++ beginning in C++2a, you are allowed to
+leave the variable argument out entirely:
 
 @smallexample
 eprintf ("success!\n")
@@ -1696,8 +1703,24 @@ eprintf ("success!\n")
 @end smallexample
 
 @noindent
-Second, the @samp{##} token paste operator has a special meaning when
-placed between a comma and a variable argument.  If you write
+Second, C++2a introduces the @code{@w{__VA_OPT__}} function macro.
+This macro may only appear in the definition of a variadic macro.  If
+the variable argument has any tokens, then a @code{@w{__VA_OPT__}}
+invocation expands to its argument; but if the variable argument does
+not have any tokens, the @code{@w{__VA_OPT__}} expands to nothing:
+
+@smallexample
+#define eprintf(format, @dots{}) \\
+  fprintf (stderr, format __VA_OPT__(,) __VA_ARGS__)
+@end smallexample
+
+@code{@w{__VA_OPT__}} is also available in GNU C and GNU C++.
+
+Historically, GNU CPP has also had another extension to handle the
+trailing comma: the @samp{##} token paste operator has a special
+meaning when placed between a comma and a variable argument.  Despite
+the introduction of @code{@w{__VA_OPT__}}, this extension remains
+supported in GNU CPP, for backward compatibility.  If you write
 
 @smallexample
 #define eprintf(format, @dots{}) fprintf (stderr, format, ##__VA_ARGS__)
@@ -1730,6 +1753,9 @@ of macro.  It may also be forbidden in open text; the standard is
 ambiguous.  We recommend you avoid using it except for its defined
 purpose.
 
+Likewise, C++ forbids @code{@w{__VA_OPT__}} anywhere outside the
+replacement list of a variadic macro.
+
 Variadic macros became a standard part of the C language with C99.  
 GNU CPP previously supported them
 with a named variable argument