Bug: [no-floating-promises] Incorrect handling of finally calls

[aleclarson]

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• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have searched for related issues and found none that matched my issue.
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Playground Link

https://typescript-eslint.io/play/#ts=4.8.3&sourceType=module&code=MYewdgzgLgBAJgUwLYgGIFczCgS3ALhgAocwAHdKQgIxBABsEBDMAShgF4A+GABQCcQSHBAQAeaP1IBzHh2KkKVGLQbM2nHgG8AUDBg4AZgvKV2u-fv4Io6fmD6DhogHTWIDAG4IiAcgAWCPT0IDAA7iD89HC+rADcejAAvjBBojAWlta29o5CIghuCABWCNh+ipThTBAwhkz0orEJ+kk6bTqIKBhYuOBEUPzoCKwuUIFgRETs3BmJoJBqLiHSfgCiYFAI1nAw4whgze2jhqQN9ACeUzPa8+AejMsgq74bWzt1Z8EXR0nxQA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Jge1oDN4OBDfMwDmtYtA4BbSshTooiaBOiRwYAL4h1QA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA

Repro Code

demoFunction(true).then(() => {
  console.log('Entered then');
}).finally(() => {
  console.log('Entered finally');
});

ESLint Config

module.exports = {
  parser: "@typescript-eslint/parser",
  rules: {
    "@typescript-eslint/no-floating-promises": ["error"],
  },
};

tsconfig

No response

Expected Result

There is no .catch call, and the rule should report it.

Actual Result

Nothing reported

Additional Info

No response

Versions

package	version
@typescript-eslint/eslint-plugin	5.31.0
@typescript-eslint/parser	5.31.0
TypeScript	4.7.4
ESLint	8.20.0
node	18.7.0

[JoshuaKGoldberg]

Hmm, we'd explicitly made this the behavior in #1603 -> #1620. But now I'm not so sure this was the right move. .finally() creates a new Promise, and the code inside it might throw an error.

console.log("a");
Promise.resolve().finally(() => {
  throw new Error("Finally!");
});
console.log("b");

a
b
Uncaught (in promise) Error: Finally!
    at <anonymous>:3:9
    at <anonymous>

cc @a-tarasyuk @bradzacher for historical context. Perhaps we should make the ignoring of .finally(...) calls behind an option for the rule?

[aleclarson]

Floating promises can't be entirely avoided, since .catch handlers can throw. That said, the real issue here is that .finally handlers are basically just sugar for…

p.finally(handle)
// …same as…
p.then(
  async (result) => {
    await handle()
    return result
  },
  async (error) => {
    await handle()
    throw error
  }
)

…and so they should be treated the same as .then calls by the no-floating-promises rule.