interop: a registry spelled on an any virtual parameter is never validated against the method's #123

(Written by Claude Code, on behalf of @jll63.)

detail::validate_method_parameter for the any and Boost.TypeErasure interop headers takes a
ParamRegistry and never compares it with the method's registry. A mismatch that every other
virtual parameter shape rejects is accepted silently — and the parameter is not inert, because it
decides the registry of a method that does not name one.

Behaviour

Parameter spells R, method is on other_registry:

Reproducer

#include <any>
#include <boost/openmethod.hpp>
#include <boost/openmethod/interop/std_any.hpp>

using namespace boost::openmethod;
struct other_registry : default_registry::with<> {};

// Passes: the mismatch is accepted.
static_assert(detail::validate_method_parameter<
    virtual_<const std::any&, default_registry>, other_registry, void>::value);

// Also passes: the spelled registry decided the method's registry.
static_assert(std::is_same_v<
    detail::method_registry<void(virtual_<const std::any&, other_registry>)>,
    other_registry>);

Both compile on GCC 13 at -std=c++17. Replacing the first parameter with a plain
virtual_<Animal&, default_registry> hard-errors at core.hpp:2330 instead.

Cause

The specializations are unconditional : std::true_type {}, with ParamRegistry deduced and then
unused:

• interop/std_any.hpp:24,29,33
• interop/boost_any.hpp:24,29,33
• interop/boost_type_erasure.hpp:98,103,108,113,118,123

Meanwhile param_registry's generic virtual_<T, Registry> pattern (core.hpp:2392) picks the
registry up — no interop header specializes param_registry — so the parameter votes in
agreed_registry and method_registry_aux takes it.

virtual_any is unaffected: its specializations match virtual_any<Any, Registry>&, a different
shape, and do compare.

Consequence

BOOST_OPENMETHOD(f, (virtual_<const std::any&, other_registry>), void);

with no registry named on the method builds f in other_registry. Overriders registered against
the default registry land in a different dispatch table, and the call reports no_overrider at run
time after initialize().

Coverage

None of the five compile_fail_*registry*mismatch* tests uses an interop shape; #116 added only
positive static_asserts (test_dispatch_std_any.cpp).