[views] Take a reference in single_view to ensure proper semantics

Author: ldionne

example/view/view.cpp

@@ -33,4 +33,9 @@ int main() {
     // The work is done here, when accessing the elements of the view.
     BOOST_HANA_RUNTIME_CHECK(hana::at_c<0>(mammals).name == "Garfield");
     BOOST_HANA_RUNTIME_CHECK(hana::at_c<1>(mammals).name == "Snoopy");
+
+    // Views have reference semantics, so we can use them to modify the
+    // original containers (unless the view applies a transformation).
+    hana::at_c<1>(mammals).name = "Beethoven";
+    BOOST_HANA_RUNTIME_CHECK(hana::at_c<2>(animals).name == "Beethoven");
 }

include/boost/hana/view.hpp

@@ -150,14 +150,13 @@ BOOST_HANA_NAMESPACE_BEGIN
         //////////////////////////////////////////////////////////////////////
         template <typename T>
         struct single_view_t {
-            T value_;
+            T& value_;
             using hana_tag = view_tag;
         };
 
         template <typename T>
-        constexpr single_view_t<typename hana::detail::decay<T>::type>
-        single_view(T&& t)
-        { return {static_cast<T&&>(t)}; }
+        constexpr single_view_t<T> single_view(T& t)
+        { return {t}; }
 
         template <typename T>
         struct is_view<single_view_t<T>> {
@@ -349,14 +348,7 @@ BOOST_HANA_NAMESPACE_BEGIN
 
         // single_view
         template <typename T, typename N>
-        static constexpr T& apply(detail::single_view_t<T>& view, N const&) {
-            static_assert(N::value == 0,
-            "trying to fetch an out-of-bounds element in a hana::single_view");
-            return view.value_;
-        }
-
-        template <typename T, typename N>
-        static constexpr T const& apply(detail::single_view_t<T> const& view, N const&) {
+        static constexpr decltype(auto) apply(detail::single_view_t<T> view, N const&) {
             static_assert(N::value == 0,
             "trying to fetch an out-of-bounds element in a hana::single_view");
             return view.value_;

test/view/single/at.cpp

@@ -15,10 +15,11 @@ using hana::test::ct_eq;
 
 int main() {
     {
-        auto single = hana::detail::single_view(ct_eq<0>{});
+        ct_eq<0> x{};
+        auto single = hana::detail::single_view(x);
         BOOST_HANA_CONSTANT_CHECK(hana::equal(
             hana::at(single, hana::size_c<0>),
-            ct_eq<0>{}
+            x
         ));
     }
 }

test/view/single/is_empty.cpp

@@ -13,7 +13,8 @@ template <int> struct undefined { };
 
 int main() {
     {
-        auto single = hana::detail::single_view(undefined<0>{});
+        undefined<0> x{};
+        auto single = hana::detail::single_view(x);
         BOOST_HANA_CONSTANT_CHECK(hana::not_(hana::is_empty(single)));
     }
 }

test/view/single/length.cpp

@@ -14,7 +14,8 @@ template <int> struct undefined { };
 
 int main() {
     {
-        auto single = hana::detail::single_view(undefined<0>{});
+        undefined<0> x{};
+        auto single = hana::detail::single_view(x);
         BOOST_HANA_CONSTANT_CHECK(hana::equal(
             hana::length(single),
             hana::size_c<1>

test/view/single/modifications.cpp

@@ -9,8 +9,10 @@ namespace hana = boost::hana;
 
 
 int main() {
-    auto view = hana::detail::single_view(3);
+    int x = 3;
+    auto view = hana::detail::single_view(x);
     hana::at_c<0>(view) = 93;
 
     BOOST_HANA_RUNTIME_CHECK(hana::at_c<0>(view) == 93);
+    BOOST_HANA_RUNTIME_CHECK(x == 93);
 }

test/view/single/unpack.cpp

@@ -16,10 +16,11 @@ int main() {
     auto f = hana::test::_injection<0>{};
 
     {
-        auto single = hana::detail::single_view(ct_eq<0>{});
+        ct_eq<0> x{};
+        auto single = hana::detail::single_view(x);
         BOOST_HANA_CONSTANT_CHECK(hana::equal(
             hana::unpack(single, f),
-            f(ct_eq<0>{})
+            f(x)
         ));
     }
 }