interface IFace<T> 
{
    void Foo<S>() where S : T;
}

class Stuff : IFace<string>
{
  // Alt+Ins, Implement interface member, explicit
}

Result:

interface IFace<T> 
{
    void Foo<S>() where S : T;
}

class Stuff : IFace<string>
{
    void IFace<string>.Foo<S>() where S : string // error CS0460: Constraints for override and explicit interface implementation methods are inherited from the base method, so they cannot be specified directly
    {
        throw new NotImplementedException();
    }
}

Do not insert 'where' clause in explicit implementation.
Also, do not suggest to implement implicitly, if it would produce incorrect constraint (for example, sealed class constraint)

Issue was closed