detail::span_convertible handles void types

fixes #202

include/boost/core/span.hpp

@@ -23,9 +23,17 @@ class span;
 
 namespace detail {
 
+template<class U, class T, class = void>
+struct span_convertible
+{
+    static constexpr bool value = false;
+};
+
 template<class U, class T>
-struct span_convertible {
+struct span_convertible<U, T, typename std::enable_if<
-    static constexpr bool value = std::is_convertible<U(*)[], T(*)[]>::value;
+    std::is_convertible<U(*)[], T(*)[]>::value>::type>
+{
+    static constexpr bool value = true;
 };
 
 template<std::size_t E, std::size_t N>

test/span_constructible_test.cpp

@@ -25,6 +25,20 @@ struct range {
     }
 };
 
+struct buffer {
+    void* data() {
+        return 0;
+    }
+
+    const void* data() const {
+        return 0;
+    }
+
+    std::size_t size() const {
+        return 0;
+    }
+};
+
 struct base { };
 
 struct derived
@@ -138,6 +152,10 @@ void test_range()
         const range<int>&>));
     BOOST_TEST_TRAIT_FALSE((std::is_constructible<boost::span<base>,
         range<derived>&>));
+    BOOST_TEST_TRAIT_FALSE((std::is_constructible<boost::span<int>,
+        buffer>));
+    BOOST_TEST_TRAIT_FALSE((std::is_constructible<boost::span<int>,
+        const buffer&>));
 }
 
 void test_initializer_list()