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

@@ -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;
   }