Wide contract const lvalue reference observer of the stored exception. NoValuePolicy::wide_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 if NoValuePolicy::wide_exception_check() returns.

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

Requires: Always available.

Complexity: Depends on NoValuePolicy::wide_exception_check().

Guarantees: None.