MultiTypeBlockVector: Use std::common_type for field_type

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 - Added a function to write nested matrices as SVG objects: `writeSVGMatrix(...)`
 
+- `MultiTypeBlockVector` uses now `std::common_type` of the entries for the `field_type`. The old `double`
+  default is only used in case of an empty `MultiTypeBlockVector`.
+
 - MINRES: The algorithm computes the preconditioned defect during the iterations. However, the initial
   defect was computed as the defect of the original/non-preconditioned system. This is now changed so
   that the initial defect is also computed as the preconditioned defect (this is also in line with GMRes).

dune/istl/multitypeblockvector.hh

@@ -11,6 +11,7 @@
 #include <dune/common/ftraits.hh>
 #include <dune/common/hybridutilities.hh>
 #include <dune/common/typetraits.hh>
+#include <dune/common/std/type_traits.hh>
 
 #include "istlexception.hh"
 
@@ -73,12 +74,16 @@ namespace Dune {
 
     /** \brief The type used for scalars
      *
-     * The current code hardwires it to 'double', which is far from nice.
-     * On the other hand, it is not clear what the correct type is.  If the MultiTypeBlockVector class
-     * is instantiated with several vectors of different field_types, what should the resulting
-     * field_type be?
+     * Use the `std::common_type_t` of the `Args`' field_type and use `nonesuch` if no implementation of
+     * `std::common_type` is provided for the given `field_type` arguments.
      */
-    typedef double field_type;
+    using field_type = Std::detected_t<std::common_type_t, typename FieldTraits< std::decay_t<Args> >::field_type...>;
+
+    // make sure that we have an std::common_type: using an assertion produces a more readable error message
+    // than a compiler template instantiation error
+    static_assert ( sizeof...(Args) == 0 or not std::is_same_v<field_type, Std::nonesuch>,
+        "No std::common_type implemented for the given field_types of the Args. Please provide an implementation and check that a FieldTraits class is present for your type.");
+
 
     /** \brief Return the number of non-zero vector entries
      *

dune/istl/test/multitypeblockvectortest.cc

@@ -10,6 +10,7 @@
 #endif
 
 #include <iostream>
+#include <complex>
 
 #include <dune/common/exceptions.hh>
 #include <dune/common/fvector.hh>
@@ -111,6 +112,16 @@ int main(int argc, char** argv) try
 
   testMultiVector(multiVectorRef);
 
+  // test field_type (std::common_type) for some combinations
+  static_assert ( std::is_same_v<typename MultiTypeBlockVector<BlockVector<FieldVector<double,3> >, BlockVector<FieldVector<double,1> > >::field_type,
+                                 double > );
+  static_assert ( std::is_same_v<typename MultiTypeBlockVector<BlockVector<FieldVector<float,3> >, BlockVector<FieldVector<float,1> > >::field_type,
+                                 float > );
+  static_assert ( std::is_same_v<typename MultiTypeBlockVector<BlockVector<FieldVector<float,3> >, BlockVector<FieldVector<double,1> > >::field_type,
+                                 double > );
+  static_assert ( std::is_same_v<typename MultiTypeBlockVector<BlockVector<FieldVector<double,3> >, BlockVector<FieldVector<std::complex<double>,1> > >::field_type,
+                                 std::complex<double> > );
+
   return 0;
 }
 catch (Dune::Exception& e)