`IsNotNil()` predicate is too strict

[maargenton]

var _, err = base64.RawURLEncoding.DecodeString("SGVsbG8gV29ybGQK@")
verify.That(t, err).IsNotNil()

Current output:

expected: value is not nil
error:    value of type 'base64.CorruptInputError' can never be nil
value:    16

Because base64.CorruptInputError is a non-nillable int64 derived type that implements the error interface and returned as an error type by DecodeString(), it should be testable with .IsNil() / .IsNotNil() and be considered not nil. Instead, .IsNotNil() flags this as a programming error because of the underlying type.

Expected behavior:

.IsNotNil(), commonly used for testing that a non-specific error is returned, should pass for any value of a non-nillable types, like numbers and strings.

[maargenton]

Resolved in v1.4.0