Sometimes _Compare is an lvalue reference type, so letting it be deduced is pretty much always wrong. (Well, less efficient than it could be, anyway.)
I did a manual audit for other offending calls (and I found none), by manually replacing _Compare __comp with a non-deducible typename __identity<_Compare>::type __comp in all the helper functions. My impression is that such a blanket change to the helpers would be unwelcome because it's so icky-looking. And perhaps also because it's an ABI break, in the sense that it would affect the mangling of the __sort specializations explicitly instantiated in src/algorithm.cpp — it's unclear to me whether we would care about that.