@NonNullByDefault
public class Example {
	public String func(List<String> list){
		return list.stream().collect(Collectors.joining(","));
	}
}

Null type safety (type annotations): The expression of type 'Collector<CharSequence,capture#of ?,String>' needs unchecked conversion to conform to 'Collector<? super @NonNull String,capture#of ?,@NonNull String>'

Unsafe interpretation of method return type as '@NonNull' based on substitution 'R=@NonNull String'. Declaring type 'Stream<T>' doesn't seem to be designed with null type annotations in mind

class java/util/stream/Collectors
joining
 (Ljava/lang/CharSequence;)Ljava/util/stream/Collector<Ljava/lang/CharSequence;*Ljava/lang/String;>;
 (Ljava/lang/CharSequence;)L1java/util/stream/Collector<L1java/lang/CharSequence;*1L1java/lang/String;>;

Null type safety (type annotations): The expression of type '@NonNull Collector<@NonNull CharSequence,@NonNull capture#of ?,String>' needs unchecked conversion to conform to 'Collector<? super @NonNull String,@NonNull capture#of ?,@NonNull String>'