[llvm] coro.retcon: Fix return type and buffer poisoning in elided coroutines (PR #208384)

[Anton Korobeynikov via llvm-commits]

asl wrote:

> So Rust allows coroutines with zero yields, apparently.

Ah, ok, nice. I'm having a PR at https://github.com/swiftlang/swift/pull/85372, let me see if I can also reproduce this with Swift.

https://github.com/llvm/llvm-project/pull/208384