Rationale (October, 2009):

The point at which std::unexpected is called is specified in _N4606_.15.5.2 [except.unexpected] paragraph 1:

If a function with an exception-specification throws an exception that is not listed in the exception-specification, the function std::unexpected() is called (_N4606_.D.6 [exception.unexpected]) immediately after completing the stack unwinding for the former function.

That is, it will be called after any local automatic objects and temporaries are destroyed and before any other objects, such as function arguments, are destroyed. (See 7.6.1.3 [expr.call] paragraph 4: “The initialization and destruction of each parameter occurs within the context of the calling function.”)

It is unclear whether std::unexpected is called before or after the destruction of function arguments, partially-constructed bases and members (when called from a constructor or destructor), etc.