Why does String's "init<T>(_ value: T) where T : LosslessStringConvertible" use a generic constraint?

[Karl]

ExistentialSpecializer (in its earliest form) was merged on September 26, 2018 (so after Swift 4.2, which was released on September 17, 2018). It's actually a relatively recent addition.

But yes, the 2 function signatures are semantically equivalent.

@Joe_Groff

Yeah, passing an existential as an argument is isomorphic to passing a generic argument with its own type variable—anything that's possible with one representation is possible on the other. We've even discussed giving them the same underlying ABI.