In C++20 variadic data can be empty. This means that BOOST_PP_VARIADIC_SIZE now returns 0 for empty data instead of 1, and that conversions to and from empty variadic data and lists or arrays, which can be empty, work properly.

test/tuple.cxx

@@ -24,6 +24,7 @@
 # if BOOST_PP_VARIADICS
 # include <boost/preprocessor/variadic/size.hpp>
 # include <boost/preprocessor/variadic/elem.hpp>
+# include <boost/preprocessor/variadic/has_opt.hpp>
 # endif
 # include <libs/preprocessor/test/test.h>
 # include <libs/preprocessor/test/tuple_elem_bug_test.cxx>
@@ -131,7 +132,16 @@ BEGIN BOOST_PP_VARIADIC_ELEM(45,BOOST_PP_TUPLE_ENUM(BOOST_PP_TUPLE_REVERSE(TUPLE
 
 // enum
 
+# if BOOST_PP_VARIADIC_HAS_OPT()
+
+BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_ENUM(TUPLE_NONE)) == 0 END
+
+# else
+
 BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_ENUM(TUPLE_NONE)) == 1 END
+
+# endif
+
 BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_ENUM(TUPLE)) == 6 END
 BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_ENUM(TUPLE_LARGE)) == 33 END
 BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_ENUM(TUPLE_VERY_LARGE)) == 64 END
@@ -189,7 +199,16 @@ BEGIN BOOST_PP_TUPLE_ELEM(33, BOOST_PP_TUPLE_PUSH_FRONT(TUPLE_LARGE, 33)) == 32
 
 #if BOOST_PP_VARIADICS_MSVC && _MSC_VER <= 1400
 
+# if BOOST_PP_VARIADIC_HAS_OPT()
+
+BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_REM(1)()) == 0 END
+
+# else
+
 BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_REM(1)()) == 1 END
+
+# endif
+
 BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_REM(33)(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32)) == 33 END
 BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_REM(64)(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63)) == 64 END
 
@@ -209,7 +228,16 @@ BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_REM()(0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
 
 // rem_ctor
 
+# if BOOST_PP_VARIADIC_HAS_OPT()
+
+BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_REM_CTOR(TUPLE_NONE)) == 0 END
+
+# else
+
 BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_REM_CTOR(TUPLE_NONE)) == 1 END
+
+# endif
+
 BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_REM_CTOR(TUPLE)) == 6 END
 BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_REM_CTOR(TUPLE_LARGE)) == 33 END
 BEGIN BOOST_PP_VARIADIC_SIZE(BOOST_PP_TUPLE_REM_CTOR(TUPLE_VERY_LARGE)) == 64 END