Add GCC's "-Wuninitialized" workaround. When initializing the pointer of flags part of an uninitialized pointer plus bit object, the other part is used uninitialized. It's harmless warning but annoying for the user.

doc/intrusive.qbk

@@ -3870,6 +3870,13 @@ to be inserted in intrusive containers are allocated using `std::vector` or `std
 
 [section:release_notes Release Notes]
 
+[section:release_notes_boost_1_67_00 Boost 1.67 Release]
+
+*  Fixed bugs:
+   *  [@https://github.com/boostorg/intrusive/issues/29 GitHub Issues #29: ['Uninitialized variable warning pointer_plus_bits.hpp]]
+
+[endsect]
+
 [section:release_notes_boost_1_65_00 Boost 1.65 Release]
 
 *  Fixed bugs:

include/boost/intrusive/pointer_plus_bits.hpp

@@ -22,6 +22,17 @@
 #  pragma once
 #endif
 
+
+//GCC reports uninitialized values when an uninitialized pointer plus bits type
+//is asigned some bits or some pointer value, but that's ok, because we don't want
+//to default initialize parts that are not being updated.
+#if defined(BOOST_GCC)
+#  if (BOOST_GCC >= 40600)
+#     pragma GCC diagnostic push
+#     pragma GCC diagnostic ignored "-Wuninitialized"
+#  endif
+#endif
+
 namespace boost {
 namespace intrusive {
 
@@ -89,6 +100,10 @@ struct pointer_plus_bits<T*, NumBits>
 } //namespace intrusive
 } //namespace boost
 
+#if defined(BOOST_GCC) && (BOOST_GCC >= 40600)
+#  pragma GCC diagnostic pop
+#endif
+
 #include <boost/intrusive/detail/config_end.hpp>
 
 #endif //BOOST_INTRUSIVE_POINTER_PLUS_BITS_HPP