This feature would allow a variable be declared at the same time that is it passed as an out parameter:

argument-value:
    out type identifier

A variable declared this way is read-only and scoped to the enclosing statement. More specifically, the scope will be the same as for a pattern-variable introduced via pattern-matching.

You should be able to use the contextual keyword var for the variable's type, in which case the specification for overload resolution would have to be modified to account for that possibility.