Reduce compilation time by using SkipParser when determining attribute type.

When using a large parser, the whole tree of parsers in instantiated with the
skip (whitespace) parser used. Then, it was instantiated again without a skip
parser, just to determine the attribute type.
This patch changes the code so the attribute type is determined *with* the
skip parser. That way, the number of template instantiations
required are halved for some use cases.

With gcc 14.2, the instantiations without the skip parser even ended up in the
binary. In that case, this patch reduces binary size.
This is most likely a compiler bug, as the usage is in decltype().

Author: Andreas Buhr

include/boost/parser/parser.hpp

@@ -374,7 +374,10 @@ namespace boost { namespace parser {
         template<typename T>
         using print_type = typename print_t<T>::type;
 
-        template<typename R, typename Parser>
+        struct null_parser
+        {};
+
+        template<typename R, typename Parser, typename SkipParser = null_parser>
         struct attribute_impl;
 
         // Utility types.
@@ -1466,9 +1469,6 @@ namespace boost { namespace parser {
                    uint32_t(flags::in_apply_parser);
         }
 
-        struct null_parser
-        {};
-
         struct skip_skipper
         {
             template<
@@ -2541,7 +2541,7 @@ namespace boost { namespace parser {
             detail::skip(first, last, skip, flags);
             using attr_t = typename detail::attribute_impl<
                 BOOST_PARSER_SUBRANGE<std::remove_const_t<Iter>, Sentinel>,
-                Parser>::type;
+                Parser, SkipParser>::type;
             try {
                 attr_t attr_ =
                     parser(first, last, context, skip, flags, success);
@@ -9671,7 +9671,7 @@ namespace boost { namespace parser {
     }
 
     namespace detail {
-        template<typename R, typename Parser>
+        template<typename R, typename Parser, typename SkipParser>
         struct attribute_impl
         {
             using parser_type = typename Parser::parser_type;
@@ -9694,7 +9694,7 @@ namespace boost { namespace parser {
                 std::declval<iterator &>(),
                 std::declval<sentinel>(),
                 std::declval<context>(),
-                detail::null_parser{},
+                SkipParser{},
                 detail::flags::gen_attrs,
                 std::declval<bool &>()));
         };