Hard to understand, but the compile error message makes this clear for me:
func funkyfunk<T: CustomStringConvertible>(_ v: T) {
print(v.description)
}
funkyfunk(Foo() as CustomStringConvertible) // <== compile error here
**error: value of protocol type 'CustomStringConvertible' cannot conform to 'CustomStringConvertible'; only struct/enum/class types can conform to protocols**
**funkyfunk(Foo() as CustomStringConvertible)**
**note: required by global function 'funkyfunk' where 'T' = 'CustomStringConvertible'**
**func funkyfunk<T: CustomStringConvertible>(_ v: T) {**
Rephrasing the first error message: Protocols cannot comform to protocols. Only struct/enum/class types can conform to protocols.
So with just a protocol type, Swift cannot call the specific appendInterpolation<Subject:...>(...), so it just call the unqualified generic one (the last one). There is no way to make string interpolation to call the specific one with just a protocol with common overloading.
But should work with specific call signature:
"\(csc: s1, default: "s1 is nil")"
(never mind, do not work with just protocol, still the same reason: protocol cannot conform to protocol)
I think it's clear now for me. 
Also, all four appendInterpolation<Subject:...>(...) are needed in order to get the right specialized String.init(describing:)