// (13,9): warning CS8620: Argument of type 'C<string, string>' cannot be used for parameter 'dictionary' of type 'C<string, string?>' in 'string? Extensions.GetOrAdd<string, string?>(C<string, string?> dictionary, string key, Func<string, string?> function)' due to differences in the nullability of reference types.
// (14,9): warning CS8620: Argument of type 'C<string, string>' cannot be used for parameter 'dictionary' of type 'C<string, string?>' in 'string? Extensions.GetOrAdd<string, string?>(C<string, string?> dictionary, string key, Func<string, string?> function)' due to differences in the nullability of reference types.
// (15,9): warning CS8620: Argument of type 'C<string, string>' cannot be used for parameter 'dictionary' of type 'C<string, string?>' in 'string? Extensions.GetOrAdd<string, string?>(C<string, string?> dictionary, string key, Func<string, string?> function)' due to differences in the nullability of reference types.