Wide contract lvalue reference observer of the stored error. NoValuePolicy::wide_error_check() is first invoked, then the reference to the error is returned. As a valid default constructed error is always present, no undefined behaviour occurs if NoValuePolicy::wide_error_check() returns.

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

Requires: Always available.

Complexity: Depends on NoValuePolicy::wide_error_check().

Guarantees: None.