Adjusted the configuration so that compiling a cuda source file ( .cu ) using clang does not set BOOST_PP_VARIADICS to 0. An 'explicit' test was also added for this situation.

test/Jamfile.v2

@@ -247,3 +247,13 @@ alias preprocessor_config
    :   
         [ run config_info.cpp ]
   ;
+  
+alias clang_cuda
+   :   
+        [ compile clang_cuda.cu 
+          : <cxxflags>--cuda-gpu-arch=sm_20
+          : clang_cuda_cu
+        ]
+  ;
+
+explicit clang_cuda ;

test/clang_cuda.cu

@@ -0,0 +1,29 @@
+# /* **************************************************************************
+#  *                                                                          *
+#  *     (C) Copyright Edward Diener 2019.
+#  *     Distributed under the Boost Software License, Version 1.0. (See
+#  *     accompanying file LICENSE_1_0.txt or copy at
+#  *     http://www.boost.org/LICENSE_1_0.txt)
+#  *                                                                          *
+#  ************************************************************************** */
+#
+# /* See http://www.boost.org for most recent version. */
+#
+# include <boost/preprocessor/cat.hpp>
+#
+# define BEGIN typedef int BOOST_PP_CAT(test_, __LINE__)[((
+# define END )==1) ? 1 : -1];
+
+#if defined(__clang__) && defined(__CUDACC__) && defined(__CUDA__)
+
+BEGIN BOOST_PP_VARIADICS == 1 END
+
+#else
+
+BEGIN 0 == 1 END
+
+#endif
+
+int main(void) {
+    return 0;
+}