std::map<const uint8_t, double>::find() warning from MISRA C++

Question

We have the following code:

class Foo {
    std::map<const uint8_t, double> m_throttle;
    // ...
    void bar(const uint8_t &msg_id)
    {
        if (m_throttle.find(msg_id) == m_throttle.end())
        {
            // Whatever
        }
    }
};

This produces the following error from the QA-C code checker when applying the MISRA C++ 2008 rules, highlighting the msg_id as the problematic token:

05-00-03 複合式が暗黙的に異なる本質型へ変換されています。

Or in English:

Complex expressions are implicitly converted to different intrinsic types.

According to the PDF here, fuller text is:

A cvalue expression shall not be implicitly converted to a different underlying type

However, the implementation of std::map::find() is just:

iterator
find(const key_type& __x)
{ return _M_t.find(__x); }

Where key_type should be uint8_t. Is this a bug in the tool, or is there something I am missing?

Note the tool uses the currently-installed gcc, apparently, which is 5.4.0.