From 0e1241d9a1f8014174087b5d24021456055949df Mon Sep 17 00:00:00 2001
From: Daniel Arndt <arndtd@ornl.gov>
Date: Thu, 28 May 2020 18:23:23 -0400
Subject: [PATCH] Fix compiling with gcc with C++20

---
 include/deal.II/base/linear_index_iterator.h | 22 ++++++++------------
 1 file changed, 9 insertions(+), 13 deletions(-)

diff --git a/include/deal.II/base/linear_index_iterator.h b/include/deal.II/base/linear_index_iterator.h
index d69845f212..f74ca0f289 100644
--- a/include/deal.II/base/linear_index_iterator.h
+++ b/include/deal.II/base/linear_index_iterator.h
@@ -260,10 +260,16 @@ public:
   }
 
   /**
-   * Inverse of operator==().
+   * Opposite of operator==().
    */
-  bool
-  operator!=(const DerivedIterator &) const;
+  template <typename OtherIterator>
+  friend typename std::enable_if<
+    std::is_convertible<OtherIterator, DerivedIterator>::value,
+    bool>::type
+  operator!=(const LinearIndexIterator &left, const OtherIterator &right)
+  {
+    return !(left == right);
+  }
 
   /**
    * Comparison operator: uses the same ordering as operator<(), but also
@@ -451,16 +457,6 @@ inline typename LinearIndexIterator<DerivedIterator, AccessorType>::pointer
 
 
 
-template <class DerivedIterator, class AccessorType>
-inline bool
-LinearIndexIterator<DerivedIterator, AccessorType>::
-operator!=(const DerivedIterator &other) const
-{
-  return !(*this == other);
-}
-
-
-
 template <class DerivedIterator, class AccessorType>
 inline bool
 LinearIndexIterator<DerivedIterator, AccessorType>::
-- 
2.39.5