I think you missed my point. I understood how you inverted explicit and implicit, you saw me explain that to someone else who asked about Promise.all(). What I

I think you missed my point. I understood how you inverted explicit and implicit, you saw me explain that to someone else who asked about Promise.all(). What I'm asking you is, how does anyone, be it the programmer, compiler, or runtime, know whether to block on qux() or not, given that it sometimes synchronously returns 1? Are you saying that in your proposal, it would become part of the calling convention that any time any function at all returns (and no 'async' keyword was used), the callee would check if a Promise was returned and if so, return a continuation promise? That is no small runtime cost. In an untyped language like JS, the cases where static analysis could determine whether a function returns a Promise or not are vanishingly few, probably about the same as the cases where a function can be inlined. Remember, anyone at any point can overwrite Array.prototype.push to be anything they want, including a function that returns a Promise. Any function that takes and calls a callback might be calling an async function. Any math or string operation could be calling .valueOf() or .toString(), which might be async.