Without this array include, compilation of the dvektor test will fail.
This has been fixed upstream in:
https://github.com/arximboldi/immer/commit/f381e4988c1562f0f64533146c9b9cb7c0823b3e

diff --git a/immer/experimental/detail/dvektor_impl.hpp b/immer/experimental/detail/dvektor_impl.hpp
index 4f69733..7a21f1f 100644
--- a/immer/experimental/detail/dvektor_impl.hpp
+++ b/immer/experimental/detail/dvektor_impl.hpp
@@ -16,6 +16,7 @@
 #include <boost/iterator/iterator_facade.hpp>
 #include <boost/smart_ptr/intrusive_ref_counter.hpp>
 
+#include <array>
 #include <cassert>
 #include <cstddef>
 #include <limits>