#nullable enable

interface IEquatable<T> {
  bool Equals(T other);    
}

interface IFoo : IEquatable<IFoo?>, IEquatable<string?>
{ }

class Foo : IFoo
{
    public bool Equals(string? other)
    {
        return true;
    }

    public bool Equals(IFoo? other)
    {
        return true;
    }
}

class C
{
    void M(IFoo foo)
    {
        foo.Equals("");
        //         ^^ CS8620
        // Argument of type 'string' cannot be used for parameter 'other' of type 'IFoo'
        // in 'bool IEquatable<IFoo?>.Equals(IFoo? other)' due to differences in the
        // nullability of reference types.
    }
}