Narrow contract lvalue reference observer of the stored exception. NoValuePolicy::narrow_exception_check() is first invoked, then the reference to the exception is returned. As a valid default constructed exception is always present, no undefined behaviour occurs unless NoValuePolicy::narrow_exception_check() does that.

Note that if exception_type is void, only a const overload returning void is present.

Requires: Always available.

Complexity: Depends on NoValuePolicy::narrow_exception_check().

Guarantees: An exception is never thrown.