type inference limitation with streams

If the explicit type argument to map is removed from the following example it is rejected by the nullness checker. It would be nice if the checker's type inference handled this case without the explicit type.

import java.util.Collections;
import java.util.List;
import java.util.Objects;
import java.util.stream.Collectors;
import org.checkerframework.checker.nullness.qual.Nullable;

class T {
  @Nullable
  static String toString(Object o) {
    return null;
  }

  public static List<String> f(List<Integer> xs) {
    return xs != null
        ? xs.stream()
            .<@Nullable String>map(T::toString)
            .filter(Objects::nonNull)
            .collect(Collectors.toList())
        : Collections.emptyList();
  }
}

If <@Nullable String> is removed:

T.java:16: error: [methodref.return.invalid] Incompatible return type
            .map(T::toString)
                 ^
  Method
    @Initialized @Nullable String toString(@Initialized @NonNull Object p0) in T
  is not a valid method reference for
    @Initialized @NonNull String apply(@Initialized @NonNull Function<@Initialized @NonNull Integer, @Initialized @NonNull String> this, @Initialized @NonNull Integer p0) in java.util.function.Function
  found   : @Initialized @Nullable String
  required: @Initialized @NonNull String
1 error

Start by compiling the reduced Java example with the Checker Framework nullness checker, both with and without the explicit <@Nullable String> type argument on map. Trace the nullness checker's inference for the stream method reference and filter; done means the version without the explicit type argument is accepted without introducing a new diagnostic.