https://issues.dlang.org/show_bug.cgi?id=21602

          Issue ID: 21602
           Summary: Invalid covariant parameter override allowed
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: major
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: qs.il.paperinik@gmail.com

interface SinkString
{
    void toString(void delegate(string) sink);
}

class Aggregate : SinkString
{
    override void toString(void delegate(string) @safe sink) @safe
    {
        sink("I'm an Aggreate!");
    }
}

Here, void delegate(string) @safe is not a super-type of void delegate(string) which it would need to be to be sound by the Liskov substitution principle. To the contrary, it allows passing @system `sink`s to an Aggregate object through a SinkString reference.

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