fix: older gcc SFINAE bug

Author: vinniefalco

include/boost/buffers/data_source.hpp

@@ -40,13 +40,12 @@ struct is_data_source
 };
 
 template<class T>
-struct is_data_source<T, detail::void_t<typename
-    std::enable_if<
+struct is_data_source<T, detail::void_t<
+    decltype(std::declval<T const&>().data())>>
+    : std::integral_constant<bool,
         std::is_nothrow_move_constructible<T>::value &&
         buffers::is_const_buffer_sequence<
-            decltype(std::declval<T const&>().data())>::value
-        >::type> >
-    : std::true_type
+            decltype(std::declval<T const&>().data())>::value>
 {
 };
 

test/unit/data_source.cpp

@@ -55,13 +55,26 @@ struct read_source
         system::error_code&);
 };
 
+// blows up in gcc 5,6,7 without workarounds
+struct gcc_banger
+{
+    template<class T,
+        typename std::enable_if<
+            is_data_source<typename std::decay<T>::type>::value,
+        int>::type = 0>
+    gcc_banger(T&&);
+    
+    gcc_banger(gcc_banger&&) = default;
+};
+
 } // (anon)
 
 BOOST_CORE_STATIC_ASSERT(  is_data_source<test_source>::value);
 BOOST_CORE_STATIC_ASSERT(! is_data_source<int>::value);
 BOOST_CORE_STATIC_ASSERT(! is_data_source<not_source1>::value);
 BOOST_CORE_STATIC_ASSERT(! is_data_source<not_source2>::value);
 BOOST_CORE_STATIC_ASSERT(! is_data_source<read_source>::value);
+BOOST_CORE_STATIC_ASSERT(! is_data_source<gcc_banger>::value);
 
 struct data_source_test
 {