Merge pull request #3352 from stan-dev/fix/3351-indexing-checks-vector-of-eigen

 Direct all std::vector assigns through size-aware version

Author: WardBrian

src/stan/model/indexing/assign.hpp

@@ -59,6 +59,7 @@ inline void assign(Tuple1&& x, Tuple2&& y, const char* name);
 template <
     typename T, typename U,
     require_t<std::is_assignable<std::decay_t<T>&, std::decay_t<U>>>* = nullptr,
+    require_all_not_std_vector_t<T, U>* = nullptr,
     math::require_all_not_tuple_t<T, U>* = nullptr>
 inline void assign(T&& x, U&& y, const char* name) {
   internal::assign_impl(x, std::forward<U>(y), name);
@@ -776,15 +777,25 @@ inline void assign(Mat1&& x, Mat2&& y, const char* name, const Idx& row_idx,
  * @param[in] y rvalue variable
  * @param[in] name name of lvalue variable
  */
-template <typename T, typename U, require_all_std_vector_t<T, U>* = nullptr,
-          require_not_t<
-              std::is_assignable<std::decay_t<T>&, std::decay_t<U>>>* = nullptr>
+template <typename T, typename U, require_all_std_vector_t<T, U>* = nullptr>
 inline void assign(T&& x, U&& y, const char* name) {
   if (unlikely(x.size() != 0)) {
     stan::math::check_size_match("assign array size", name, x.size(),
                                  "right hand side", y.size());
   }
-  if (std::is_rvalue_reference<U&&>::value) {
+
+  if constexpr (std::is_assignable_v<std::decay_t<T>&, std::decay_t<U>>) {
+    if (is_stan_scalar_v<value_type_t<T>> || x.size() == 0) {
+      x = std::forward<U>(y);
+      return;
+    }
+  }
+
+  // If we've made it this far, we need x to have elements to assign to,
+  // and we know this is either a no-op or x has size 0, which we treat as
+  // a wildcard that can take any size.
+  x.resize(y.size());
+  if constexpr (std::is_rvalue_reference_v<U&&>) {
     for (size_t i = 0; i < y.size(); ++i) {
       assign(x[i], std::move(y[i]), name);
     }

src/test/unit/model/indexing/assign_test.cpp

@@ -1197,3 +1197,17 @@ TEST(model_indexing, tuples_non_trivially_assignable) {
       },
       A, B);
 }
+
+TEST(model_indexing, stdvec_of_eigvec_errors) {
+  using stan::model::assign;
+  using stan::model::index_min_max;
+
+  std::vector<Eigen::VectorXd> x(1);
+  x[0] = Eigen::VectorXd(2);
+  x[0] << 1, 2;
+  std::vector<Eigen::VectorXd> y(1);
+  y[0] = Eigen::VectorXd(3);
+  y[0] << 3, 4, 5;
+
+  test_throw_ia(x, y);
+}