Nested array support

Author: andrjohns

stan/math/prim/fun/size_mvt.hpp

@@ -26,6 +26,11 @@ int64_t size_mvt(const ScalarT& /* unused */) {
   throw std::invalid_argument("size_mvt passed to an unrecognized type.");
 }
 
+template <typename ScalarT, require_stan_scalar_t<ScalarT>* = nullptr>
+int64_t size_mvt(const std::vector<ScalarT>& /* unused */) {
+  return 1;
+}
+
 template <typename MatrixT, require_matrix_t<MatrixT>* = nullptr>
 int64_t size_mvt(const MatrixT& /* unused */) {
   return 1;
@@ -36,6 +41,11 @@ int64_t size_mvt(const std::vector<MatrixT>& x) {
   return x.size();
 }
 
+template <typename StdVectorT, require_std_vector_t<StdVectorT>* = nullptr>
+int64_t size_mvt(const std::vector<StdVectorT>& x) {
+  return x.size();
+}
+
 }  // namespace math
 }  // namespace stan
 #endif

stan/math/prim/fun/vector_seq_view.hpp

@@ -6,6 +6,15 @@
 #include <vector>
 
 namespace stan {
+namespace internal {
+template <typename T>
+using is_matrix_or_std_vector
+  = math::disjunction<is_matrix<T>, is_std_vector<T>>;
+
+template <typename T>
+using is_scalar_container = math::conjunction<is_matrix_or_std_vector<T>,
+                                              is_stan_scalar<value_type_t<T>>>;
+}
 
 /**
  * This class provides a low-cost wrapper for situations where you either need
@@ -33,7 +42,7 @@ class vector_seq_view;
  * @tparam T the type of the underlying Vector
  */
 template <typename T>
-class vector_seq_view<T, require_matrix_t<T>> {
+class vector_seq_view<T, require_t<internal::is_scalar_container<T>>> {
  public:
   explicit vector_seq_view(const T& m) : m_(m) {}
   static constexpr auto size() { return 1; }
@@ -46,19 +55,13 @@ class vector_seq_view<T, require_matrix_t<T>> {
 
   template <typename C = T, require_st_autodiff<C>* = nullptr>
   inline auto val(size_t /* i */) const noexcept {
-    return m_.val();
+    return value_of(m_);
   }
 
  private:
   const ref_type_t<T> m_;
 };
 
-namespace internal {
-template <typename T>
-using is_matrix_or_std_vector
-    = math::disjunction<is_matrix<T>, is_std_vector<T>>;
-}
-
 /**
  * This class provides a low-cost wrapper for situations where you either need
  * an Eigen Vector or RowVector or a std::vector of them and you want to be