Do not test boost::throws() under -fsanitize=undefined

.travis.yml

@@ -264,7 +264,7 @@ install:
 script:
   - |-
     echo "using $TOOLSET : : $COMPILER ;" > ~/user-config.jam
-  - ./b2 -j3 libs/system/test toolset=$TOOLSET cxxstd=$CXXSTD ${UBSAN:+cxxflags=-fsanitize=undefined} ${UBSAN:+cxxflags=-fno-sanitize-recover=undefined} ${UBSAN:+linkflags=-fsanitize=undefined}
+  - ./b2 -j3 libs/system/test toolset=$TOOLSET cxxstd=$CXXSTD ${UBSAN:+cxxflags=-fsanitize=undefined} ${UBSAN:+cxxflags=-fno-sanitize-recover=undefined} ${UBSAN:+linkflags=-fsanitize=undefined} ${UBSAN:+define=UBSAN=1}
 
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test/error_code_test.cpp

@@ -22,6 +22,7 @@
 #include <cstring>
 #include <functional>
 #include <boost/cerrno.hpp>
+#include <boost/config/pragma_message.hpp>
 
 //  Although using directives are not the best programming practice, testing
 //  with a boost::system using directive increases use scenario coverage.
@@ -246,8 +247,17 @@ int main( int, char ** )
   BOOST_TEST( econd.message() != "" );
   BOOST_TEST( econd.message().substr( 0, 13) != "Unknown error" );
 
+#if !defined(UBSAN)
+
+  // the current implementation of boost::throws() relies on undefined behavior
   test_throws_usage();
 
+#else
+
+  BOOST_PRAGMA_MESSAGE("Skipping test_throws_usage() due to UBSAN");
+
+#endif
+
 #ifdef BOOST_WINDOWS_API
   std::cout << "Windows tests...\n";
   // these tests probe the Windows errc decoder