[integral_constant] Unsigned integral constants are not EuclideanRings

Fixes #240
boostorg · Nov 17, 2016 · be252aa · be252aa
1 parent 9fe9d2f
commit be252aa
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diff --git a/include/boost/hana/detail/integral_constant.hpp b/include/boost/hana/detail/integral_constant.hpp
@@ -112,9 +112,11 @@ BOOST_HANA_NAMESPACE_BEGIN
     //!     value<integral_constant<T, v>>() == v // of type T
     //! @endcode
     //!
-    //! 2. `Comparable`, `Orderable`, `Logical`, `Monoid`, `Group`, `Ring`, and `EuclideanRing`, `Hashable`\n
+    //! 2. `Comparable`, `Orderable`, `Logical`, `Monoid`, `Group`, `Ring`, and `Hashable`\n
     //! Those models are exactly those provided for `Constant`s, which are
     //! documented in their respective concepts.
+    //!
+    //! 3. `EuclideanRing` for signed types
 #ifdef BOOST_HANA_DOXYGEN_INVOKED
     template <typename T, T v>
     struct integral_constant {

diff --git a/include/boost/hana/div.hpp b/include/boost/hana/div.hpp
@@ -72,10 +72,11 @@ BOOST_HANA_NAMESPACE_BEGIN
     };
 
     //////////////////////////////////////////////////////////////////////////
-    // Model for integral data types
+    // Model for signed integral data types
     //////////////////////////////////////////////////////////////////////////
     template <typename T>
     struct div_impl<T, T, when<std::is_integral<T>::value &&
+                               std::is_signed<T>::value &&
                                !std::is_same<T, bool>::value>> {
         template <typename X, typename Y>
         static constexpr decltype(auto) apply(X&& x, Y&& y)

diff --git a/include/boost/hana/fwd/concept/euclidean_ring.hpp b/include/boost/hana/fwd/concept/euclidean_ring.hpp
@@ -93,17 +93,21 @@ BOOST_HANA_NAMESPACE_BEGIN
     //! `hana::integral_constant`
     //!
     //!
-    //! Free model for non-boolean integral data types
-    //! ----------------------------------------------
-    //! A data type `T` is integral if `std::is_integral<T>::%value` is true.
-    //! For a non-boolean integral data type `T`, a model of `EuclideanRing`
+    //! Free model for non-boolean signed integral data types
+    //! -----------------------------------------------------
+    //! A data type `T` is integral if `std::is_integral<T>::%value` is true,
+    //! and it is signed if `std::is_signed<T>::%value` is true. For a
+    //! non-boolean signed integral data type `T`, a model of `EuclideanRing`
     //! is automatically defined by using the `Ring` model provided for
     //! arithmetic data types and setting
     //! @code
     //!     div(x, y) = (x / y)
     //!     mod(x, y)  = (x % y)
     //! @endcode
     //!
+    //! Such a model cannot be provided for unsigned integral types, because
+    //! the fact that overflow wraps breaks the laws of `EuclideanRing`.
+    //!
     //! @note
     //! The rationale for not providing an EuclideanRing model for `bool` is
     //! the same as for not providing Monoid, Group and Ring models.

diff --git a/include/boost/hana/mod.hpp b/include/boost/hana/mod.hpp
@@ -72,10 +72,11 @@ BOOST_HANA_NAMESPACE_BEGIN
     };
 
     //////////////////////////////////////////////////////////////////////////
-    // Model for integral data types
+    // Model for signed integral data types
     //////////////////////////////////////////////////////////////////////////
     template <typename T>
     struct mod_impl<T, T, when<std::is_integral<T>::value &&
+                               std::is_signed<T>::value &&
                                !std::is_same<T, bool>::value>> {
         template <typename X, typename Y>
         static constexpr decltype(auto) apply(X&& x, Y&& y)

diff --git a/test/issues/github_240.cpp b/test/issues/github_240.cpp
@@ -0,0 +1,27 @@
+// Copyright Louis Dionne 2013-2016
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+
+#include <boost/hana/concept/euclidean_ring.hpp>
+#include <boost/hana/integral_constant.hpp>
+#include <boost/hana/not.hpp>
+
+#include <support/cnumeric.hpp>
+namespace hana = boost::hana;
+
+
+static_assert(!hana::EuclideanRing<unsigned short>{}, "");
+static_assert(!hana::EuclideanRing<unsigned int>{}, "");
+static_assert(!hana::EuclideanRing<unsigned long>{}, "");
+static_assert(!hana::EuclideanRing<unsigned long long>{}, "");
+
+template <typename T, T v>
+using minimal_constant = ::cnumeric_t<T, v>;
+static_assert(!hana::EuclideanRing<minimal_constant<unsigned short, 10>>{}, "");
+static_assert(!hana::EuclideanRing<minimal_constant<unsigned int, 10>>{}, "");
+static_assert(!hana::EuclideanRing<minimal_constant<unsigned long, 10>>{}, "");
+static_assert(!hana::EuclideanRing<minimal_constant<unsigned long long, 10>>{}, "");
+
+static_assert(!hana::EuclideanRing<hana::integral_constant<unsigned int, 10>>{}, "");
+
+int main() { }