contracts in interfaces: do they work, do I have to do something to enable checking of contracts ?

[Emil via Digitalmars-d-learn]

I am trying my hand at contracts and they work fine in plain 
functions and in methods, but I can't make them work in 
interfaces.

https://dlang.org/spec/interface.html#interface-contracts
$ dmd --version
DMD64 D Compiler v2.081.1
Copyright (C) 1999-2018 by The D Language Foundation, All Rights 
Reserved written by Walter Bright


/////////////////////////
void main()
{
    WillBloop test = new WillBloop();
    // also tried
    // Blooper test = new WillBloop();
    // it compiles but the in contract does not seem enforced
    test.limited(-1); // this does not croak

    auto no_interface = new NoInterface();
    no_interface.limited(-10); // this works as expected
}

interface Blooper
{
    void limited(int some_other_name)
    in
    {
        assert(some_other_name > 0, "asssert failed in 
interface");
    }
}

class WillBloop : Blooper
{
    void limited(int a_name)    {
        import std.stdio : writeln;
        writeln(a_name);
    }
}

class NoInterface
{
    void limited(int another_name)
    in
    {
        assert(another_name > 0, "assert failed in NoInterface");
    }
    do
    {
        import std.stdio: writeln;
        writeln(another_name);
    }
}