Surprising `error: [mustcall:methodref.receiver] Incompatible receiver type`

The analyser complains about this method:

    private static <T> List<String> toStringList(Collection<T> list)
    {
        if (list == null)
            return null;
        return list.stream().map(Object::toString).collect(Collectors.toList());
    }

with

    [javac] /home/jlewandowski/dev/cassandra/c18190-ecj/src/java/org/apache/cassandra/db/virtual/LocalRepairTables.java:460: error: [mustcall:methodref.receiver] Incompatible receiver type
    [javac]         return list.stream().map(Object::toString).collect(Collectors.toList());
    [javac]                                  ^
    [javac]   found   : @MustCall Object
    [javac]   required: T[ extends @MustCallUnknown Object super @MustCall Void]
    [javac]   Consequence: method in @MustCall Object
    [javac]     @MustCall String toString(@MustCall Object this)
    [javac]   is not a valid method reference for method in @MustCallUnknown Function<T[ extends @MustCallUnknown Object super @MustCall Void], @MustCall String>
    [javac]     @MustCall String apply(@MustCall Function<T[ extends @MustCallUnknown Object super @MustCall Void], @MustCall String> this, T[ extends @MustCallUnknown Object super @MustCall Void] p0)

I must admit I do not understand why it complains :)

Start with the toStringList method in LocalRepairTables.java at line 460 and reproduce the reported [mustcall:methodref.receiver] diagnostic using the shown Java code. Trace how the analyser types Object::toString and the generic Collection stream; done means explaining the diagnostic and determining whether the analyser or the method reference needs correction.