Fix #374: Support nested vector type conversions (#692)

vector_conversion's element conversion used Cast_Helper directly, which
only performs exact type matching. For nested vectors like
std::vector<std::vector<double>>, each inner element is a
std::vector<Boxed_Value> that needs its own conversion to
std::vector<double>. Added a recursive convert_vector_element helper
that detects std::vector value types and converts them from
std::vector<Boxed_Value> before falling back to Cast_Helper for
leaf types.

Co-authored-by: leftibot <leftibot@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

include/chaiscript/dispatchkit/type_conversions.hpp

@@ -498,6 +498,29 @@ namespace chaiscript {
                                                                                                                func);
   }
 
+  namespace detail {
+    template<typename T>
+    struct is_std_vector : std::false_type {};
+
+    template<typename T, typename A>
+    struct is_std_vector<std::vector<T, A>> : std::true_type {};
+
+    template<typename T>
+    T convert_vector_element(const Boxed_Value &bv) {
+      if constexpr (is_std_vector<T>::value) {
+        const auto &inner = Cast_Helper<const std::vector<Boxed_Value> &>::cast(bv, nullptr);
+        T result;
+        result.reserve(inner.size());
+        for (const Boxed_Value &elem : inner) {
+          result.push_back(convert_vector_element<typename T::value_type>(elem));
+        }
+        return result;
+      } else {
+        return Cast_Helper<T>::cast(bv, nullptr);
+      }
+    }
+  } // namespace detail
+
   template<typename To>
   Type_Conversion vector_conversion() {
     auto func = [](const Boxed_Value &t_bv) -> Boxed_Value {
@@ -506,7 +529,7 @@ namespace chaiscript {
       To vec;
       vec.reserve(from_vec.size());
       for (const Boxed_Value &bv : from_vec) {
-        vec.push_back(detail::Cast_Helper<typename To::value_type>::cast(bv, nullptr));
+        vec.push_back(detail::convert_vector_element<typename To::value_type>(bv));
       }
 
       return Boxed_Value(std::move(vec));

unittests/compiled_tests.cpp

@@ -1806,6 +1806,35 @@ TEST_CASE("push_back on script vector with vector_conversion") {
         == "hello,world");
 }
 
+TEST_CASE("vector of vectors conversion (issue #374)") {
+  chaiscript::ChaiScript_Basic chai(create_chaiscript_stdlib(), create_chaiscript_parser());
+
+  auto m = std::make_shared<chaiscript::Module>();
+  chaiscript::bootstrap::standard_library::vector_type<std::vector<double>>("VectorDouble", *m);
+  chaiscript::bootstrap::standard_library::vector_type<std::vector<std::vector<double>>>("VectorVectorDouble", *m);
+  m->add(chaiscript::vector_conversion<std::vector<double>>());
+  m->add(chaiscript::vector_conversion<std::vector<std::vector<double>>>());
+  chai.add(m);
+
+  chai.add(chaiscript::fun([](const std::vector<std::vector<double>> &v) -> double {
+    double sum = 0;
+    for (const auto &inner : v) {
+      for (const auto d : inner) {
+        sum += d;
+      }
+    }
+    return sum;
+  }), "sum_nested");
+
+  CHECK(chai.eval<double>("sum_nested([[1.0, 2.0], [3.0, 4.0]])") == Approx(10.0));
+
+  CHECK(chai.eval<bool>(
+    "auto v = VectorVectorDouble();"
+    "v = [[1.0, 2.0], [3.0, 4.0]];"
+    "v.size() == 2"
+  ));
+}
+
 // Regression test for issue #607: AST_Node_Trace must be a complete type
 // when used in eval_error's std::vector<AST_Node_Trace> call_stack member.
 // This failed to compile with C++20 on clang/libc++ when AST_Node_Trace