Add todos for convenient resize compiler errors.

dune/matrix-vector/genericvectortools.hh

@@ -48,6 +48,7 @@ void truncate(Vector& v, const BitVector& tr) {
 }
 
 //! Resize vector recursively to size of given vector/matrix
+// EXPERIMENTAL: Its implementation is still subject to change.
 // Note: We need the rvalue reference here, because we might obtain a
 // temporary recursively, e.g. in std::bitset the operator[] yields a
 // std::bitset::reference temporary due to its specific implementation.
@@ -114,11 +115,13 @@ auto getSize(const SizeProvider& sizeProvider) {
 }
 
 /// compile-time helper traits
+// TODO these are not satifsying, because e.g. for custom matrices that
+// do not specialiaze the MatrixTraits properly, the error messages are
+// not very helpful. In particular they may trigger a misleading static_assert.
 template <class M>
 constexpr auto isMatrix() {
   return Std::bool_constant<MatrixTraits<M>::isMatrix>();
 }
-
 template <class V>
 constexpr auto isVector() {
   // Note: At the time this comment is written, the only use of
@@ -156,6 +159,8 @@ struct ScalarSwitch<Vector,
   //       vector to be resized.
   template <class Resizeable>
   static void resize(Resizeable& v, const Vector& sizeProvider) {
+    // TODO this assert may also be triggered when isMatrix/isVector do not
+    //      work properly, e.g. due to a missing MatrixTraits specialization.
     static_assert(not IsNumber<Resizeable>::value, "SizeProvider seems to have nesting depth that is not applicable to given vector type.");
 
     using namespace Hybrid;
@@ -208,6 +213,7 @@ protected:
   // priority tag forward helper
   template <class SizeProviderRow>
   static decltype(auto) subSizeProvider(SizeProviderRow&& row) {
+    // TODO add static assert that fails or warns if subSizeProvider is not a matrix for a matrix or a vector for a vector
     return This::subSizeProvider(std::forward<SizeProviderRow>(row), PriorityTag<42>());
   }
 };