diff --git a/include/mio/detail/mmap.ipp b/include/mio/detail/mmap.ipp
index a813fc3..a068891 100644
--- a/include/mio/detail/mmap.ipp
+++ b/include/mio/detail/mmap.ipp
@@ -418,6 +418,25 @@ void basic_mmap<AccessMode, ByteT>::swap(basic_mmap& other)
     }
 }
 
+template<access_mode AccessMode, typename ByteT>
+template<access_mode A>
+typename std::enable_if<A == access_mode::write, void>::type
+basic_mmap<AccessMode, ByteT>::conditional_sync()
+{
+    // This is invoked from the destructor, so not much we can do about
+    // failures here.
+    std::error_code ec;
+    sync(ec);
+}
+
+template<access_mode AccessMode, typename ByteT>
+template<access_mode A>
+typename std::enable_if<A == access_mode::read, void>::type
+basic_mmap<AccessMode, ByteT>::conditional_sync()
+{
+    // noop
+}
+
 template<access_mode AccessMode, typename ByteT>
 bool operator==(const basic_mmap<AccessMode, ByteT>& a,
         const basic_mmap<AccessMode, ByteT>& b)
diff --git a/include/mio/mmap.hpp b/include/mio/mmap.hpp
index fd18d2b..2d18cd9 100644
--- a/include/mio/mmap.hpp
+++ b/include/mio/mmap.hpp
@@ -370,18 +370,11 @@ private:
      * if it's `read`, but since the destructor cannot be templated, we need to
      * do SFINAE in a dedicated function, where one syncs and the other is a noop.
      */
-    template<access_mode A = AccessMode,
-            typename = typename std::enable_if<A == access_mode::write>::type>
-    void conditional_sync()
-    {
-        // This is invoked from the destructor, so not much we can do about
-        // failures here.
-        std::error_code ec;
-        sync(ec);
-    }
-
     template<access_mode A = AccessMode>
-    typename std::enable_if<A == access_mode::read, void>::type conditional_sync() {}
+    typename std::enable_if<A == access_mode::write, void>::type
+    conditional_sync();
+    template<access_mode A = AccessMode>
+    typename std::enable_if<A == access_mode::read, void>::type conditional_sync();
 };
 
 template<access_mode AccessMode, typename ByteT>