[0.85] Types: Annotate Promise<void> returns on async functions - #1913
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Summary: `flow-api-translator` assumes that a function without an explicit return type annotation returns `void`. That isn't true of `async` functions, which always return a `Promise`, so our generated `.d.ts` declares seven async functions as returning `void` - including the public `Server#end()`, `Watcher#watch()`, `Watcher#close()` and `DependencyGraph#end()`. TypeScript consumers awaiting those get `await-thenable` from typescript-eslint, or quietly don't await at all. I've fixed the translator upstream in facebook/flow#9486, but 0.85.x pins `flow-api-translator` 0.36.1 and I'd rather not bump the toolchain on a release branch, so this annotates the return types at source instead. It's a no-op for Flow, which already infers `Promise<void>` in every case here, and it's what we want in the source regardless. `main` will pick the translator fix up with the next version bump, after which these annotations are still correct, just no longer load-bearing. To find the full set I translated every file the generator covers with 0.35.0 patched and unpatched and diffed the output - these seven declarations, across five files, are all of them on this branch. Changelog: ``` - **[Fix]**: Types: async methods including `Server#end`, `Watcher#watch`/`#close` and `DependencyGraph#end` are declared as returning `Promise<void>` rather than `void` ``` Test plan: ``` yarn run build-ts-defs # updates exactly the five .d.ts files, no other churn yarn typecheck # No errors! yarn typecheck-ts yarn jest packages/metro/src/Server/__tests__/Server-test.js packages/metro-file-map/src/__tests__ packages/metro-file-map/src/watchers/__tests__ ```
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Picks #1896 to 0.85.x
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