This is being emitted with [GCC 10.2.1](https://packages.debian.org/bullseye/g++-arm-linux-gnueabihf), in some CI jobs: i.e https://github.com/bitcoin/bitcoin/pull/25972/checks?check_run_id=12423693655, or https://github.com/bitcoin/bitcoin/pull/25972/checks?check_run_id=12423693651 (where we actually turn on all our warning flags with depends builds).
```bash
In file included from test/miniscript_tests.cpp:19:
./script/miniscript.h:1197:71: error: comparison of integers of different signs: 'typename iterator_traits<int *>::difference_type' (aka 'int') and 'const uint32_t' (aka 'const unsigned int') [-Werror,-Wsign-compare]
                    return std::count(subs.begin(), subs.end(), true) >= node.k;
                           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~
test/miniscript_tests.cpp:346:34: note: in instantiation of function template specialization 'miniscript::Node<CPubKey>::IsSatisfiable<(lambda at test/miniscript_tests.cpp:346:48)>' requested here
        bool satisfiable = node->IsSatisfiable([](const Node&) { return true; });
                                 ^
1 error generated.
```

cc @darosior 
{
        
        
        return std::move(*TreeEvalMaybe<Result>(std::move(root_state),
            std::forward<DownFn>(downfn),
            [&upfn](State&& state, const Node& node, Span<Result> subs) {
                Result res{upfn(std::move(state), node, subs)};
                return std::optional<Result>(std::move(res));
            }
        ));
    }
    /** Like TreeEval, but without downfn or State type.
     *  upfn takes (const Node&, Span<Result>) and returns Result. */
    template<typename Result, typename UpFn>
    Result TreeEval(UpFn upfn) const
    {
        struct DummyState {};
        return std::move(*TreeEvalMaybe<Result>(DummyState{},
            [](DummyState, const Node&, size_t) { return DummyState{}; },
            [&upfn](DummyState, const Node& node, Span<Result> subs) {
                Result res{upfn(node, subs)};
                return std::optional<Result>(std::move(res));
            }
        ));
    }
    
    friend int Compare(const Node<Key>& node1, const Node<Key>& node2)