Add safety assertion

include/boost/container/string.hpp

@@ -234,6 +234,7 @@ class basic_string_base
       //Access and copy (to avoid UB) the first byte of the union to know if the
       //active representation is short or long
       short_header hdr;
+      BOOST_STATIC_ASSERT((sizeof(short_header) == 1));
       *(unsigned char*)&hdr = *(unsigned char*)&this->members_.m_repr;
       return hdr.is_short != 0;
    }