diff --git a/include/boost/openmethod/core.hpp b/include/boost/openmethod/core.hpp
index fa551ff3..aef7690e 100644
--- a/include/boost/openmethod/core.hpp
+++ b/include/boost/openmethod/core.hpp
@@ -738,6 +738,14 @@ namespace detail {
 template<class...>
 struct use_class_aux;
 
+// Classes must be complete when they are registered. Without this check, an
+// incomplete class fails in the type traits that compute the inheritance map.
+template<class T, typename = void>
+constexpr bool is_complete = false;
+
+template<class T>
+constexpr bool is_complete<T, std::void_t<decltype(sizeof(T))>> = true;
+
 template<class Registry, class Class, typename... Bases>
 struct use_class_aux<Registry, mp11::mp_list<Class, Bases...>> :
     std::conditional_t<
@@ -826,6 +834,10 @@ using use_classes_tuple_type = boost::mp11::mp_apply<
 //! @see [Registries and Policies](xref:ROOT:registries_and_policies.adoc)
 template<class... Classes>
 class use_classes {
+    static_assert(
+        (detail::is_complete<Classes> && ...),
+        "use_classes: a class is incomplete");
+
     detail::use_classes_tuple_type<Classes...> tuple;
 };
 
@@ -1218,6 +1230,10 @@ inline auto final_virtual_ptr(Arg&& obj) {
 //! @see [Performance](xref:ROOT:performance.adoc)
 template<class Class, class Registry, typename>
 class virtual_ptr {
+    static_assert(
+        std::is_class_v<Class>,
+        "virtual_ptr<> takes a class or a smart pointer: virtual_ptr<Node>, "
+        "not virtual_ptr<Node*>");
 
     using traits = virtual_traits<Class&, Registry>;
 
@@ -2389,17 +2405,24 @@ struct validate_method_parameter<
         false_t<Class, Registry, MethodRegistry>, "registry mismatch");
 };
 
-template<class Class, class Registry>
+// A `virtual_ptr` passed by non-const lvalue reference cannot bind to the
+// `virtual_ptr` of a derived class, so it cannot have overriders for one, and
+// the dispatch treats it as the class of the object. By rvalue reference, it is
+// not a virtual parameter at all.
+template<class Class, class Registry, class MethodRegistry>
 struct validate_method_parameter<
-    virtual_ptr<Class, Registry>&, Registry, void> : std::true_type {
-    static_assert(check_affinity<Class>::value);
+    virtual_ptr<Class, Registry>&, MethodRegistry, void> : std::false_type {
+    static_assert(
+        false_t<Class, Registry, MethodRegistry>,
+        "virtual_ptr<> must be passed by value or by const reference");
 };
 
 template<class Class, class Registry, class MethodRegistry>
 struct validate_method_parameter<
-    virtual_ptr<Class, Registry>&, MethodRegistry, void> : std::false_type {
+    virtual_ptr<Class, Registry>&&, MethodRegistry, void> : std::false_type {
     static_assert(
-        false_t<Class, Registry, MethodRegistry>, "registry mismatch");
+        false_t<Class, Registry, MethodRegistry>,
+        "virtual_ptr<> must be passed by value or by const reference");
 };
 
 template<class Class, class Registry>
@@ -3138,9 +3161,55 @@ struct validate_overrider_parameter<
 template<class T>
 struct validate_overrider_parameter<T, T, void> : std::true_type {};
 
+// Whether an overrider parameter would grant write access to an object that the
+// method parameter grants read access to only. `P` and `Q` are the classes the
+// two parameters designate, cv-qualified: the referent of a reference or a
+// pointer, the `element_type` of a `virtual_ptr`. Without this check, the
+// overrider is rejected by a `static_cast` or a `dynamic_cast` deep inside the
+// thunk.
+template<class P, class Q>
+constexpr bool discards_const = std::is_const_v<P> && !std::is_const_v<Q>;
+
+template<class T>
+using designated_class = std::remove_pointer_t<std::remove_reference_t<T>>;
+
+// Whether two `virtual_ptr`s, one in a method, the other in an overrider, hold
+// the same kind of pointer: both a plain pointer, or both the same smart
+// pointer template. Without this check, a plain `virtual_ptr` in the method and
+// a `shared_virtual_ptr` in the overrider fail on a conversion inside the thunk.
+template<class T1, class R1, class T2, class R2>
+constexpr auto same_pointer_kind() -> bool {
+    if constexpr (IsSmartPtr<T1, R1> != IsSmartPtr<T2, R2>) {
+        return false;
+    } else if constexpr (!IsSmartPtr<T1, R1>) {
+        return true;
+    } else {
+        return std::is_same_v<
+            typename virtual_traits<T1, R1>::template rebind<
+                typename T2::element_type>,
+            T2>;
+    }
+}
+
+// Whether an overrider returning `From` can implement a method returning `To`.
+template<typename From, typename To>
+constexpr bool returns_compatible =
+    std::is_void_v<To> ? std::is_void_v<From> : std::is_convertible_v<From, To>;
+
 template<class T1, class R1, class T2>
 struct validate_overrider_parameter<virtual_<T1, R1>, T2, void> :
-    std::true_type {};
+    std::true_type {
+    using P = designated_class<T1>;
+    using Q = designated_class<T2>;
+
+    // Only between classes of the same hierarchy: an interop parameter such as
+    // `virtual_<const std::any&>` rejects a mutable overrider parameter itself.
+    static_assert(
+        !std::conjunction_v<
+            std::bool_constant<discards_const<P, Q>>,
+            std::is_base_of<std::remove_cv_t<P>, std::remove_cv_t<Q>>>,
+        "overrider parameter discards const qualifier");
+};
 
 template<class T1, class R1, class T2, class R2>
 struct validate_overrider_parameter<virtual_<T1, R1>, virtual_<T2, R2>, void> :
@@ -3156,6 +3225,14 @@ template<class T1, class R, class T2, class R2>
 struct validate_overrider_parameter<
     virtual_ptr<T1, R>, virtual_ptr<T2, R2>, void> : std::true_type {
     static_assert(std::is_same_v<R, R2>, "registry mismatch");
+    static_assert(
+        same_pointer_kind<T1, R, T2, R2>(),
+        "different virtual_ptr<> pointer kinds");
+    static_assert(
+        !discards_const<
+            typename virtual_ptr<T1, R>::element_type,
+            typename virtual_ptr<T2, R2>::element_type>,
+        "overrider parameter discards const qualifier");
     using C1 = virtual_type<virtual_ptr<T1, R>, R>;
     using C2 = virtual_type<virtual_ptr<T2, R>, R>;
     static_assert(
@@ -3170,6 +3247,14 @@ struct validate_overrider_parameter<
     const virtual_ptr<T1, R>&, const virtual_ptr<T2, R2>&, void> :
     std::true_type {
     static_assert(std::is_same_v<R, R2>, "registry mismatch");
+    static_assert(
+        same_pointer_kind<T1, R, T2, R2>(),
+        "different virtual_ptr<> pointer kinds");
+    static_assert(
+        !discards_const<
+            typename virtual_ptr<T1, R>::element_type,
+            typename virtual_ptr<T2, R2>::element_type>,
+        "overrider parameter discards const qualifier");
     using C1 = virtual_type<const virtual_ptr<T1, R>&, R>;
     using C2 = virtual_type<const virtual_ptr<T2, R>&, R>;
     static_assert(
@@ -3183,6 +3268,14 @@ template<class T1, class R, class T2, class R2>
 struct validate_overrider_parameter<
     virtual_ptr<T1, R>&&, virtual_ptr<T2, R2>&&, void> : std::true_type {
     static_assert(std::is_same_v<R, R2>, "registry mismatch");
+    static_assert(
+        same_pointer_kind<T1, R, T2, R2>(),
+        "different virtual_ptr<> pointer kinds");
+    static_assert(
+        !discards_const<
+            typename virtual_ptr<T1, R>::element_type,
+            typename virtual_ptr<T2, R2>::element_type>,
+        "overrider parameter discards const qualifier");
     using C1 = virtual_type<virtual_ptr<T1, R>&&, R>;
     using C2 = virtual_type<virtual_ptr<T2, R>&&, R>;
     static_assert(
@@ -3272,6 +3365,9 @@ auto method<Id, ReturnType(Parameters...), Registry>::
         (validate_overrider_parameter<Parameters, OverriderParameters>::value &&
             ...),
         "virtual_ptr category mismatch");
+    static_assert(
+        returns_compatible<OverriderReturn, ReturnType>,
+        "overrider return type is not convertible to method return type");
 
     return Overrider(
         detail::parameter_traits<Parameters, Registry>::template cast<
diff --git a/test/compile_fail_classes_incomplete.cpp b/test/compile_fail_classes_incomplete.cpp
new file mode 100644
index 00000000..b77dc988
--- /dev/null
+++ b/test/compile_fail_classes_incomplete.cpp
@@ -0,0 +1,23 @@
+// Copyright (c) 2017-2026 Jean-Louis Leroy
+// Distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// Expected diagnostic, as a CMake regex (see CMakeLists.txt).
+// expected-error: use_classes: a class is incomplete
+
+#include <boost/openmethod.hpp>
+
+struct Animal {
+    virtual ~Animal() = default;
+};
+
+struct Dog;
+
+// Without the check, this fails in the type traits that compute the
+// inheritance relationships.
+BOOST_OPENMETHOD_CLASSES(Animal, Dog);
+
+int main() {
+    return 0;
+}
diff --git a/test/compile_fail_overrider_discards_const_reference.cpp b/test/compile_fail_overrider_discards_const_reference.cpp
new file mode 100644
index 00000000..b0122ff9
--- /dev/null
+++ b/test/compile_fail_overrider_discards_const_reference.cpp
@@ -0,0 +1,31 @@
+// Copyright (c) 2017-2026 Jean-Louis Leroy
+// Distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// Expected diagnostic, as a CMake regex (see CMakeLists.txt).
+// expected-error: overrider parameter discards const qualifier
+
+#include <boost/openmethod.hpp>
+
+using namespace boost::openmethod;
+
+struct Animal {
+    virtual ~Animal() = default;
+};
+
+struct Dog : Animal {};
+
+BOOST_OPENMETHOD_CLASSES(Animal, Dog);
+
+BOOST_OPENMETHOD(poke, (virtual_<const Animal&>), void);
+
+// The method gives read access to the animal, the overrider asks for write
+// access. Without the check, this fails inside the thunk, on a `dynamic_cast`
+// that casts away constness.
+BOOST_OPENMETHOD_OVERRIDE(poke, (Dog&), void) {
+}
+
+int main() {
+    return 0;
+}
diff --git a/test/compile_fail_overrider_discards_const_virtual_ptr.cpp b/test/compile_fail_overrider_discards_const_virtual_ptr.cpp
new file mode 100644
index 00000000..6054a2f2
--- /dev/null
+++ b/test/compile_fail_overrider_discards_const_virtual_ptr.cpp
@@ -0,0 +1,31 @@
+// Copyright (c) 2017-2026 Jean-Louis Leroy
+// Distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// Expected diagnostic, as a CMake regex (see CMakeLists.txt).
+// expected-error: overrider parameter discards const qualifier
+
+#include <boost/openmethod.hpp>
+
+using namespace boost::openmethod;
+
+struct Animal {
+    virtual ~Animal() = default;
+};
+
+struct Dog : Animal {};
+
+BOOST_OPENMETHOD_CLASSES(Animal, Dog);
+
+BOOST_OPENMETHOD(poke, (virtual_ptr<const Animal>), void);
+
+// The method gives read access to the animal, the overrider asks for write
+// access. Without the check, this fails inside the thunk, on a `dynamic_cast`
+// that casts away constness.
+BOOST_OPENMETHOD_OVERRIDE(poke, (virtual_ptr<Dog>), void) {
+}
+
+int main() {
+    return 0;
+}
diff --git a/test/compile_fail_overrider_pointer_kind.cpp b/test/compile_fail_overrider_pointer_kind.cpp
new file mode 100644
index 00000000..ae4e9fc1
--- /dev/null
+++ b/test/compile_fail_overrider_pointer_kind.cpp
@@ -0,0 +1,32 @@
+// Copyright (c) 2017-2026 Jean-Louis Leroy
+// Distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// Expected diagnostic, as a CMake regex (see CMakeLists.txt).
+// expected-error: different virtual_ptr<> pointer kinds
+
+#include <boost/openmethod.hpp>
+#include <boost/openmethod/interop/std_shared_ptr.hpp>
+
+using namespace boost::openmethod;
+
+struct Animal {
+    virtual ~Animal() = default;
+};
+
+struct Dog : Animal {};
+
+BOOST_OPENMETHOD_CLASSES(Animal, Dog);
+
+BOOST_OPENMETHOD(poke, (virtual_ptr<Animal>), void);
+
+// A `virtual_ptr<Dog>` converts to a `virtual_ptr<Animal>`, so the overrider
+// finds the method, but the method has no `shared_ptr` to give it. Without the
+// check, this fails on a conversion inside the thunk.
+BOOST_OPENMETHOD_OVERRIDE(poke, (shared_virtual_ptr<Dog>), void) {
+}
+
+int main() {
+    return 0;
+}
diff --git a/test/compile_fail_overrider_return_type.cpp b/test/compile_fail_overrider_return_type.cpp
new file mode 100644
index 00000000..1091eea6
--- /dev/null
+++ b/test/compile_fail_overrider_return_type.cpp
@@ -0,0 +1,33 @@
+// Copyright (c) 2017-2026 Jean-Louis Leroy
+// Distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// Expected diagnostic, as a CMake regex (see CMakeLists.txt).
+// expected-error: overrider return type is not convertible to method return type
+
+#include <string>
+
+#include <boost/openmethod.hpp>
+
+using namespace boost::openmethod;
+
+struct Animal {
+    virtual ~Animal() = default;
+};
+
+struct Dog : Animal {};
+
+BOOST_OPENMETHOD_CLASSES(Animal, Dog);
+
+BOOST_OPENMETHOD(weight, (virtual_ptr<Animal>), int);
+
+// Without the check, this fails on the return statement of the thunk that
+// calls the overrider.
+BOOST_OPENMETHOD_OVERRIDE(weight, (virtual_ptr<Dog>), std::string) {
+    return "heavy";
+}
+
+int main() {
+    return 0;
+}
diff --git a/test/compile_fail_virtual_ptr_by_lvalue_reference.cpp b/test/compile_fail_virtual_ptr_by_lvalue_reference.cpp
new file mode 100644
index 00000000..60c7dae7
--- /dev/null
+++ b/test/compile_fail_virtual_ptr_by_lvalue_reference.cpp
@@ -0,0 +1,33 @@
+// Copyright (c) 2017-2026 Jean-Louis Leroy
+// Distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// Expected diagnostic, as a CMake regex (see CMakeLists.txt).
+// expected-error: virtual_ptr<> must be passed by value or by const reference
+
+#include <boost/openmethod.hpp>
+#include <boost/openmethod/initialize.hpp>
+
+using namespace boost::openmethod;
+
+struct Animal {
+    virtual ~Animal() = default;
+};
+
+struct Dog : Animal {};
+
+BOOST_OPENMETHOD_CLASSES(Animal, Dog);
+
+// A non-const reference cannot bind to the `virtual_ptr` of a derived class,
+// and the dispatch would take `virtual_ptr` itself for the class of the object.
+BOOST_OPENMETHOD(poke, (virtual_ptr<Animal>&), void);
+
+int main() {
+    initialize();
+    Dog dog;
+    virtual_ptr<Animal> p(dog);
+    poke(p);
+
+    return 0;
+}
diff --git a/test/compile_fail_virtual_ptr_by_rvalue_reference.cpp b/test/compile_fail_virtual_ptr_by_rvalue_reference.cpp
new file mode 100644
index 00000000..d55fc881
--- /dev/null
+++ b/test/compile_fail_virtual_ptr_by_rvalue_reference.cpp
@@ -0,0 +1,31 @@
+// Copyright (c) 2017-2026 Jean-Louis Leroy
+// Distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// Expected diagnostic, as a CMake regex (see CMakeLists.txt).
+// expected-error: virtual_ptr<> must be passed by value or by const reference
+
+#include <boost/openmethod.hpp>
+#include <boost/openmethod/initialize.hpp>
+
+using namespace boost::openmethod;
+
+struct Animal {
+    virtual ~Animal() = default;
+};
+
+struct Dog : Animal {};
+
+BOOST_OPENMETHOD_CLASSES(Animal, Dog);
+
+// By rvalue reference, a `virtual_ptr` is not a virtual parameter.
+BOOST_OPENMETHOD(poke, (virtual_ptr<Animal>&&), void);
+
+int main() {
+    initialize();
+    Dog dog;
+    poke(virtual_ptr<Animal>(dog));
+
+    return 0;
+}
diff --git a/test/compile_fail_virtual_ptr_to_pointer.cpp b/test/compile_fail_virtual_ptr_to_pointer.cpp
new file mode 100644
index 00000000..29b07213
--- /dev/null
+++ b/test/compile_fail_virtual_ptr_to_pointer.cpp
@@ -0,0 +1,27 @@
+// Copyright (c) 2017-2026 Jean-Louis Leroy
+// Distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// Expected diagnostic, as a CMake regex (see CMakeLists.txt).
+// expected-error: virtual_ptr<> takes a class or a smart pointer
+
+#include <boost/openmethod.hpp>
+
+using namespace boost::openmethod;
+
+struct Animal {
+    virtual ~Animal() = default;
+};
+
+BOOST_OPENMETHOD_CLASSES(Animal);
+
+// `virtual_ptr` is itself a pointer: the parameter is `virtual_ptr<Animal>`.
+BOOST_OPENMETHOD(poke, (virtual_ptr<Animal*>), void);
+
+int main() {
+    Animal animal;
+    poke(&animal);
+
+    return 0;
+}