Improved error messaging: Cannot disown nullptr (as_unique_ptr).

include/pybind11/smart_holder_poc.h

@@ -134,6 +134,10 @@ struct smart_holder {
   }
 
   void ensure_use_count_1(const char* context) const {
+    if (vptr.get() == nullptr) {
+      throw std::runtime_error(std::string("Cannot disown nullptr (") +
+                               context + ").");
+    }
     if (vptr.use_count() != 1) {
       throw std::runtime_error(std::string("Cannot disown use_count != 1 (") +
                                context + ").");

tests/core/smart_holder_poc_test.cpp

@@ -317,3 +317,10 @@ TEST_CASE("error_disowned_holder", "[E]") {
   hld.as_unique_ptr<int>();
   REQUIRE_THROWS_WITH(hld.lvalue_ref<int>(), "Disowned holder (lvalue_ref).");
 }
+
+TEST_CASE("error_cannot_disown_nullptr", "[E]") {
+  auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
+  hld.as_unique_ptr<int>();
+  REQUIRE_THROWS_WITH(hld.as_unique_ptr<int>(),
+                      "Cannot disown nullptr (as_unique_ptr).");
+}