[Understanding] Classes and delegate inheritance vs function pointers

[Q. Schroll]

Say I have a class hierarchy like this:
   class Base { }
   class Derived : Base { }
A Derived object cannot be referenced as a Base object, but as a 
const(Base) object. That makes sense to me.

One can replace Base by a @system delegate type (SysDG) and 
Derived by a @safe delegate type (SafeDG) and it works the same 
way: a SafeDG object cannot be referenced as a SysDG object, but 
as a const(SysDG) object.

However, if I try that with function pointers instead of 
delegates (SysFP, SafeFP), for some reason, a SafeFP cannot be 
referenced as a const(SysFP).
This makes no sense in my head. Is there some reason I'm unable 
to see?

Example code is here: https://run.dlang.io/is/zSNArx