IsErr compares two values (got & expect) and returns nil if the comparison is true, an ErrFail if the comparison is false, or an ErrInvalid if the comparison is invalid. IsErr also takes an optional argument, a comparator, that changes how the comparison is made.

Is & IsErr are similar but different:

Is(...)     // Should only be called within a Terst(...) call
IsErr(...)  // A standalone comparator, the same as Is, just without the automatic reporting

IsErr is referenced in 1 repository