/// Well-formed checking. If a `WellFormedLoc` is provided,
/// then it will be used to eprform HIR-based wf checking
/// after an error occurs, in order to generate a more precise error span.
/// Well-formed checking. If a `WellFormedLoc` is provided,
/// then it will be used to eprform HIR-based wf checking
/// after an error occurs, in order to generate a more precise error span.