Reading from map requiring entangled assignment leaks on LHS.

[This](http://fanyare.tcardenas.me:5600/?gist=98400d8a0c95b022839ff3cf883bad30) correctly doesn't compile, but then [this](http://fanyare.tcardenas.me:5600/?gist=e246d4245e751e6531d8e7c75423d80f) does and crashes.
