DIP proposal is here: https://github.com/FeepingCreature/DIPs/blob/DIP-inclusive-in-contracts/DIPs/1NNN-MB.md

Preview PR is here: https://github.com/dlang/dmd/pull/11465

Previous discussion is here: https://forum.dlang.org/thread/mjcppoaykiwqegkfrerb@forum.dlang.org

Summary in code:

class Parent {
  void foo(int i) in (i >= 0) { }
}

class Child : Parent {
  // Why does this say i >= 5? I don't know, maybe the parent contract used to be i >= 5.
  // Maybe it's a typo. The point is:
  void foo(int i) in (i >= 5) {
    int value = array[i - 5]; // RangeError.
    // Why? The actual in-contract for `foo` is
    // `in (i >= 0 /* from parent */ || i >= 5)`.
    // IOW `in (i >= 0)`. Our `in` does nothing.
    // This is bad and should be changed.
  }
}

Proposed change: if the child method's in-contract is tighter than the parent, ie. when the child in-contract as written fails when the parent passes (which is not allowed to happen in Liskov), then raise a runtime error.