Half-Lifted Conversions

Given

```csharp
public struct S1
{
    public static implicit operator C(S1 s)
    {
        return new C();
    }
    public static implicit operator S2?(S1 s)
    {
        return null;
    }
}

public struct S2 { }

public class C { }
```

Existing compilers allow the following

```csharp
        Func func1 = x => x;

        Func func2 = x => x;
```
 
But neither is justified by the specification.  It would be nice if the specification were revised to allow these.