Work around a bug in GCC 5-10

GCC 5-10 incorrectly complain about our nested classes being private.
So, make them public.

Author: gennaroprota

include/boost/static_string/config.hpp

@@ -279,4 +279,11 @@ using basic_string_view =
 #define BOOST_STATIC_STRING_USE_STD_FORMAT
 #endif
 
+#if defined(__GNUC__) && (__GNUC__ >= 5) && (__GNUC__ <= 10) && !defined(__clang__)
+// Workaround for GCC complaining about nested classes being private.
+#define BOOST_STATIC_STRING_GCC_NESTED_CLASS_WORKAROUND public:
+#else
+#define BOOST_STATIC_STRING_GCC_NESTED_CLASS_WORKAROUND
+#endif
+
 #endif

include/boost/static_string/static_string.hpp

@@ -322,6 +322,8 @@ class static_string_base
   using pointer = value_type*;
   using const_pointer = const value_type*;
 
+BOOST_STATIC_STRING_GCC_NESTED_CLASS_WORKAROUND
+
   struct size
   {
     class basic_static_string
@@ -388,6 +390,8 @@ class static_string_base<0, CharT, Traits>
   using value_type = typename Traits::char_type;
   using pointer = value_type*;
 
+BOOST_STATIC_STRING_GCC_NESTED_CLASS_WORKAROUND
+
   struct size
   {
     class basic_static_string