Seven async runtimes produce four different outputs for the same fire-and-forget program

Sources: primary, HN discussion
Summary: Brown University's Cognitive Engineering Lab identifies nine observable design dimensions in async/await, grouped as start of life, end of life and cancellation, and compares seven runtimes: Asyncio, C#, JavaScript, Tokio, Smol, Trio and Swift. Running three variations of one small program across seven runtimes produced four different outputs, and no two runtimes agreed on all three variations.
Why it matters: Async semantics carried from one language to another are not portable at the level of what a program prints.