elle.Option: Build void options via nullptr.

Author: mefyl

src/elle/Option.hxx

@@ -131,7 +131,19 @@ namespace elle
 
       template <typename T, typename A>
       std::enable_if_t<
-        !std::is_same<std::remove_cv_reference_t<T>, Head>::value,
+        std::is_same<std::remove_cv_reference_t<T>, std::nullptr_t>::value &&
+        std::is_same<Head, void>::value,
+        void>
+      _emplace(A&& value)
+      {
+        this->_index = Index;
+      }
+
+      template <typename T, typename A>
+      std::enable_if_t<
+        !std::is_same<std::remove_cv_reference_t<T>, Head>::value &&
+        (!std::is_same<std::remove_cv_reference_t<T>, std::nullptr_t>::value ||
+         !std::is_same<Head, void>::value),
         void>
       _emplace(A&& value)
       {

tests/elle/option.cc

@@ -390,10 +390,17 @@ static
 void
 void_member()
 {
-  elle::Option<void, int> x(42);
-  BOOST_TEST(x.is<int>());
-  BOOST_TEST(!x.is<void>());
-  BOOST_TEST(x.get<int>() == 42);
+  {
+    elle::Option<void, int> x(42);
+    BOOST_TEST(x.is<int>());
+    BOOST_TEST(!x.is<void>());
+    BOOST_TEST(x.get<int>() == 42);
+  }
+  {
+    elle::Option<void, int> x(nullptr);
+    BOOST_TEST(!x.is<int>());
+    BOOST_TEST(x.is<void>());
+  }
 }
 
 ELLE_TEST_SUITE()