Use dup on Containers with const Elements

[Q. Schroll via Digitalmars-d-learn]

Cases to consider: Arrays and AAs with const(T) Elements, where T 
is a value or a reference type respectively.

     class C { }
     struct S { }

     const(S)[]  varr;
     const(C)[]  carr;
     const(S)[S] vaav;
     const(C)[S] caav;
     const(S)[C] vaac;
     const(C)[C] caac;

     pragma(msg, typeof(varr.dup));
     pragma(msg, typeof(carr.dup!(const C))); // (1)
     pragma(msg, typeof(vaav.dup));
     pragma(msg, typeof(caav.dup));
     pragma(msg, typeof(vaac.dup));
     pragma(msg, typeof(caac.dup));

yields

S[]         // ok
const(C)[]  // ok
const(S)[S] // (2)
const(C)[S] // ok
const(S)[C] // ok
const(C)[C] // ok

Questions:
(1) Why do I have to specify the type here? Why does inference 
fail?
(2) Why not just S[S]?
The copy of a const S is a S so why is the copy of a (const S, 
const S)-pair not just (S, S)?