std::unordered_map::find using a type different than the Key type?

I have an unordered_map that uses a string-type as a key:

std::unordered_map<string, value> map;

A std::hash specialization is provided for string, as well as a suitable operator==.

Now I also have a "string view" class, which is a weak pointer into an existing string, avoiding heap allocations:

class string_view {
    string *data;
    size_t begin, len;
    // ...
};  

Now I'd like to be able to check if a key exists in the map using a string_view object. Unfortunately, std::unordered_map::find takes a Key argument, not a generic T argument.

(Sure, I can "promote" one to a string, but that causes an allocation I'd like to avoid.)

What I would've liked instead was something like

template<class Key, class Value>
class unordered_map
{
    template<class T> iterator find(const T &t);
};

which would require operator==(T, Key) and std::hash<T>() to be suitably defined, and would return an iterator to a matching value.

Is there any workaround?