fix: suppress GCC false-positive -Wmaybe-uninitialized in tuple_rule

GCC 7+ at -O3 emits false -Wmaybe-uninitialized from
variant2 union storage when deeply inlining constexpr
functions. Wrap the affected parse_sequence code in
tuple_rule.hpp with a diagnostic pragma.

fix #979

Author: alandefreitas

include/boost/url/detail/config.hpp

@@ -213,13 +213,6 @@
 #define BOOST_URL_MAX_SIZE ((std::size_t)UINT32_MAX - 1)
 #endif
 
-// noinline attribute
-#ifdef BOOST_GCC
-#define BOOST_URL_NO_INLINE [[gnu::noinline]]
-#else
-#define BOOST_URL_NO_INLINE
-#endif
-
 // libstdcxx copy-on-write strings
 #ifndef BOOST_URL_COW_STRINGS
 #if defined(BOOST_LIBSTDCXX_VERSION) && (BOOST_LIBSTDCXX_VERSION < 60000 || (defined(_GLIBCXX_USE_CXX11_ABI) && _GLIBCXX_USE_CXX11_ABI == 0))

include/boost/url/grammar/impl/tuple_rule.hpp

@@ -160,6 +160,12 @@ struct parse_sequence
     }
 };
 
+// See error_types.hpp for details (#979)
+#if defined(BOOST_GCC) && BOOST_GCC >= 70000
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+
 // returns a value_type
 template<class R0, class... Rn>
 struct parse_sequence<false, R0, Rn...>
@@ -218,7 +224,7 @@ struct parse_sequence<false, R0, Rn...>
         system::result<void> rv =
             grammar::parse(
                 it, end, get<Ir>(rn));
-        if( !rv )
+        if( rv.has_error() )
         {
             v = rv.error();
             return;
@@ -273,6 +279,10 @@ struct parse_sequence<false, R0, Rn...>
     }
 };
 
+#if defined(BOOST_GCC) && BOOST_GCC >= 70000
+#pragma GCC diagnostic pop
+#endif
+
 } // detail
 
 template<