[swift-evolution] [discussion] What generics feature would this be?

[Mathew Huusko V]

Curious what part of generics the below would fall under? This hit me by
surprise/makes even non-generic/Self-depending protocols un-interchangeable
with classes..

```
protocol Special {}

func doWithAndReturn<S: Special>(_ special: S) -> S { ... }

let special: Special = ...

// "error: Generics parameter 'S' could not be inferred"
// works fine when 'Special' is a class
let newSpecial: Special = doWithAndReturn(special)
```
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