void M(IEnumerable<string?> values)
{
    // CS8620 Argument of type 'StringComparer' cannot be used for parameter 'comparer' of type
    // 'IEqualityComparer<string?>' in 'IEnumerable<string?> Enumerable.Distinct<string?>(
    // IEnumerable<string?> source, IEqualityComparer<string?> comparer)' due to differences in
    // the nullability of reference types.
    //              ↓
    values.Distinct(StringComparer.CurrentCultureIgnoreCase);
}