[solverfactory] Defer UMFPack check, as non-double field type triggers a compile error.

In the rewrite the vector types of UMPack were checked, similar to the
other direct solvers. This does not work anymore, as recent changes in
UMFPack now lead to a compiler error if the vectors are not available,
i.e. when using a real-type different from double. This change now
(re-)introduces a SFINAE check to ensure that the types are compatible.

dune/istl/umfpack.hh

@@ -794,6 +794,16 @@ namespace Dune {
     enum { value = true };
   };
 
+  namespace UMFPackImpl {
+    template<class OpTraits, class=void> struct isValidBlock : std::false_type{};
+    template<class OpTraits> struct isValidBlock<OpTraits,
+      std::enable_if_t<
+           std::is_same_v<Impl::UMFPackDomainType<typename OpTraits::matrix_type>, typename OpTraits::domain_type>
+        && std::is_same_v<Impl::UMFPackRangeType<typename OpTraits::matrix_type>, typename OpTraits::range_type>
+        && std::is_same_v<typename FieldTraits<typename OpTraits::domain_type::field_type>::real_type, double>
+        && std::is_same_v<typename FieldTraits<typename OpTraits::range_type::field_type>::real_type, double>
+      >> : std::true_type {};
+  }
   DUNE_REGISTER_SOLVER("umfpack",
                        [](auto opTraits, const auto& op, const Dune::ParameterTree& config)
                        -> std::shared_ptr<typename decltype(opTraits)::solver_type>
@@ -809,11 +819,7 @@ namespace Dune {
                            // check if UMFPack<M>* is convertible to
                            // InverseOperator*. This checks compatibility of the
                            // domain and range types
-                           if constexpr (std::is_convertible_v<UMFPack<M>*,
-                                         Dune::InverseOperator<typename OpTraits::domain_type,
-                                         typename OpTraits::range_type>*>
-                                         // check whether the Matrix field_type is double or complex<double>
-                                         && std::is_same_v<typename FieldTraits<M>::real_type, double>){
+                           if constexpr (UMFPackImpl::isValidBlock<OpTraits>::value) {
                              const M& mat = opTraits.getMatOrThrow(op);
                              int verbose = config.get("verbose", 0);
                              return std::make_shared<Dune::UMFPack<M>>(mat,verbose);