How to attach function attributes to delegate type?

[Q. Schroll]

For any type constructors like const, I can use ConstOf!T to get 
`T` with const attached. For a delegate/function type DG, e.g. 
int delegate(int), how can I get the @safe version of that type, 
i.e. int delegate(int) @safe?

I tried

     alias SafeOf(DG) = DG @safe;

but it didn't compile.

The case is not @safe-specific; it's the same for all function 
attributes.