ibiatiroler@… — Wed, 14 Mar 2018 12:31:43 GMT

Additional information - I.e., tested with "official" MinGW-GCC 6.3.0

Source-file "test.cpp":

#include <boost/type_traits/integral_constant.hpp>
extern bool test(boost::mpl::bool_<true> const&);
bool result = test(boost::true_type());

Compile will result in following warning:

> gcc -I /Temp/SDK/boost -O2 -Wstrict-aliasing=2 -c test.cpp
In file included from test.cpp:2:0:
/Temp/SDK/boost/boost/type_traits/integral_constant.hpp: In instantiation of 'boost::integral_constant<bool, val>::operator const mpl_::bool_<val>&() const [with bool val = true]':
test.cpp:6:38:   required from here
/Temp/SDK/boost/boost/type_traits/integral_constant.hpp:93:29: warning: type-punning to incomplete type might break strict-aliasing rules [-Wstrict-aliasing]
          return dereference(reinterpret_cast<const mpl::bool_<val>*>(&data));
                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This will not occur, if I do apply following patch (as unified diff); I.e., using an helper-variable (ptr) and removing dereference-helper, which is (IMHO) no longer needed ...

Index: boost/type_traits/integral_constant.hpp
===================================================================
--- boost/type_traits/integral_constant.hpp
+++ boost/type_traits/integral_constant.hpp
@@ -55,17 +55,12 @@
       typedef T value_type;
       typedef integral_constant<T, val> type;
       static const T value = val;
-      //
-      // This helper function is just to disable type-punning
-      // warnings from GCC:
-      //
-      template <class U>
-      static U& dereference(U* p) { return *p; }
       operator const mpl::integral_c<T, val>& ()const
       {
-         static const char data[sizeof(long)] = { 0 };
-         return dereference(reinterpret_cast<const mpl::integral_c<T, val>*>(&data));
+         static const long data = 0;
+         const void* ptr = &data;
+         return *(reinterpret_cast<const mpl::integral_c<T, val>*>(ptr));
       }
       BOOST_CONSTEXPR operator T()const { return val; }
    };
@@ -80,17 +75,12 @@
       typedef bool value_type;
       typedef integral_constant<bool, val> type;
       static const bool value = val;
-      //
-      // This helper function is just to disable type-punning
-      // warnings from GCC:
-      //
-      template <class T>
-      static T& dereference(T* p) { return *p; }
       operator const mpl::bool_<val>& ()const
       {
          static const char data = 0;
-         return dereference(reinterpret_cast<const mpl::bool_<val>*>(&data));
+         const void* ptr = &data;
+         return *(reinterpret_cast<const mpl::bool_<val>*>(ptr));
       }
       BOOST_CONSTEXPR operator bool()const { return val; }
    };

Is my suggested patch a suitable fix and does replace the the dereference-helper (which does not work for me on -Wstrict-aliasing=2)?

Best regards from Salzburg, Markus

status changed; resolution set

John Maddock — Thu, 29 Mar 2018 10:49:34 GMT

status new → closed
resolution → fixed

Apologies for not getting to this sooner, fixed in https://github.com/boostorg/type_traits/commit/4c3706113a5a130dcc66a13da6134e5ede122e33 Not sure if it will make the next release or not as it's already in beta.

Boost C++ Libraries: Ticket #13479: GCC-warning: break strict-aliasing rules in

status changed; resolution set