From 86d8a736ef52eb6ddf8a9a0ea99ff9f3652cff77 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Franz=20P=C3=B6schel?= <franz.poeschel@gmail.com>
Date: Thu, 25 May 2023 13:36:33 +0200
Subject: [PATCH] Help older compilers deal with this

---
 include/openPMD/Iteration.hpp | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/include/openPMD/Iteration.hpp b/include/openPMD/Iteration.hpp
index 5eb5f3a18c..b19359b8c2 100644
--- a/include/openPMD/Iteration.hpp
+++ b/include/openPMD/Iteration.hpp
@@ -126,18 +126,27 @@ namespace internal
  */
 class Iteration : public CustomHierarchy
 {
-    template <typename T, typename T_key, typename T_container>
-    friend class Container;
+public:
+    using IterationIndex_t = uint64_t;
+
+    /*
+     * Some old compilers have trouble with befriending the entire Container
+     * template here, so we restrict it
+     * to Container<Iteration, IterationIndex_t>, more is not needed anyway.
+     *
+     * E.g. on gcc-7:
+     * > error: specialization of 'openPMD::Container<openPMD::CustomHierarchy>'
+     * > after instantiation
+     * >      friend class Container;
+     */
+    friend class Container<Iteration, IterationIndex_t>;
     friend class Series;
     friend class WriteIterations;
     friend class SeriesIterator;
 
-public:
     Iteration(Iteration const &) = default;
     Iteration &operator=(Iteration const &) = default;
 
-    using IterationIndex_t = uint64_t;
-
     /**
      * @tparam  T   Floating point type of user-selected precision (e.g. float,
      * double).