From 301d13755a3fd0829aafcb6db3eb98f0b31216ba Mon Sep 17 00:00:00 2001
From: Max Kahnt <max.kahnt@fu-berlin.de>
Date: Thu, 28 Sep 2017 17:02:59 +0200
Subject: [PATCH] Workaround for enable_if not providing a type...

... when it does not depend on the function template parameter
(experienced with clang).
---
 dune/matrix-vector/genericvectortools.hh | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/dune/matrix-vector/genericvectortools.hh b/dune/matrix-vector/genericvectortools.hh
index 9c15e8a..ed8ac89 100644
--- a/dune/matrix-vector/genericvectortools.hh
+++ b/dune/matrix-vector/genericvectortools.hh
@@ -183,18 +183,20 @@ protected:
   /// Note: We use PriorityTags to enforce trial order w.r.t. failure fallback.
   /// Additionally we use it to circumvent ambiguous overloading.
   /// Call subSizeProvider without PriorityTag via the priority tag forward helper for standard behaviour.
+  /// We use AlwaysTrue to silence errors about enable_if not providing a type when it does not depend
+  /// on the function template parameter.
   // ... from a sparse/dynamic matrix
-  template <class SizeProviderRow, typename = std::enable_if_t<isMatrix<Vector>() and not isTupleOrDerived<Vector>()>>
+  template <class SizeProviderRow, typename = std::enable_if_t<isMatrix<Vector>() and not isTupleOrDerived<Vector>() and AlwaysTrue<SizeProviderRow>::value>>
   static decltype(auto) subSizeProvider(SizeProviderRow&& row, PriorityTag<21>) {
     return *row.begin();
   }
   // .. from a tuple like/static matrix
-  template <class SizeProviderRow, typename = std::enable_if_t<isMatrix<Vector>() and isTupleOrDerived<Vector>()>>
+  template <class SizeProviderRow, typename = std::enable_if_t<isMatrix<Vector>() and isTupleOrDerived<Vector>() and AlwaysTrue<SizeProviderRow>::value>>
   static decltype(auto) subSizeProvider(SizeProviderRow&& row, PriorityTag<20>) {
     return row[Indices::_0];
   }
   // ... from a vector
-  template <class SizeProviderRow, typename = std::enable_if_t<isVector<Vector>()>>
+  template <class SizeProviderRow, typename = std::enable_if_t<isVector<Vector>() and AlwaysTrue<SizeProviderRow>::value>>
   static decltype(auto) subSizeProvider(SizeProviderRow&& row, PriorityTag<10>) {
     return row;
   }
-- 
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