JSDoc unexpected type error when spreading array containing different shapes of same type

[Arkellys]

🔎 Search Terms

"typescript jsdoc error spread union", "jsdoc error spread type different shape"

🕗 Version & Regression Information

I was unable to test this on prior versions because the TS playground doesn't handle JSDoc

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.4.5#code/FASwdgLgpgTgZgQwMZQAQGED2YAmIIjbqoDewqFqSAXKgM4QzgDmA3OZQEYD8tYUAN1jsAvsFCRYiFBmx4C2AEKkOFJL1T8hMdpVSdaWMHBBtgY4BACeABzRGTzVAF5ZufITDKAPm-mf0cSRsBlRoUNcACmDjU0NsRwBtAF0AShcAPlREABs6KHE4AFcwJAUwVBgoCCKYMABBOgBJCAAeABVbKAzIhBhmWk67VMGulQoqmrrUPrMLAHp51ABRAA87MqgceiKkFDo6SygGSMSSfVpzmlQAIiQb1BFHtPZwiFPzg1IL7+u7h6egJe4jep2AAEhPpcqLR-o9HgAaCFQ75fK6w+7wwHAYHARYrdZQTbbRAgHJHE5nZ6pViofEABRgmDsMGsqAA5Jx2agQHRUABbXl0FgU95UtFYl50paM5mwNmc7m8gVCkV4pYAVX4hOJ2QQZNQAHd8AALVAAKQAygARTBIBE7PbHPnGiBm9qW0VgyE-dG3TGApHggB0ocmtQazTFEJ9Er+AcRMZRyZh-oBWIhaRxNKAA

💻 Code

The playground link contains the TS version, which work as expected, the below is the JSDoc equivalent throwing the unexpected type error:

/**
 * @typedef {object} ConditionC
 * @property {string} c
 * @property {never} [b]
 */

/**
 * @typedef {object} ConditionB
 * @property {never} [c]
 * @property {Config} b
 */

/**
 * @typedef {ConditionB | ConditionC} Config
 */

/**
 * @param {Config[]} config
 * @returns {boolean}
 */
const test = (config) => false;

/**
 * @template Type
 * @param {Type} arg
 * @returns {Type}
 */

function returnAsIt(arg) {
  return arg;
}

// Expected success
test([{ b: { c: "c" } }]);
test([{ b: { b: { c: "c" } } }]);

test([
  { b: { c: "c" } },
  { b: { b: { c: "c" } } }
]);

// Expected fail
test([{ }]); // Property 'b' is missing
test([{ b: {} }]); // Property 'b' is missing

// Unexpected fail
test([
  { b: { c: "c" } },
  ...returnAsIt([
    { b: { c: "c" } },
    { b: { b: { c: "c" } } }
  ])
]);

🙁 Actual behavior

With union type and recursion, when using a function that returns the same type as its input, an unexpected type error appear in some cases.

These pass:

test([
  { b: { c: "c" } },
  ...returnAsIt([
    { b: { c: "c" } }
    { b: { c: "c" } }
  ])
]);

test([
  { b: { c: "c" } },
  ...returnAsIt([
     { b: { b: { c: "c" } } }
     { b: { b: { c: "c" } } }
  ])
]);

test([
  { b: { c: "c" } },
  { b: { b: { c: "c" } } },
  ...returnAsIt([
    { b: { c: "c" } },
    { b: { b: { c: "c" } } }
  ])
]);

test([
  { b: { c: "c" } },
  ...([
    { b: { c: "c" } },
    { b: { b: { c: "c" } } }
  ])
]);

But this fail:

test([
  { b: { c: "c" } },
  ...returnAsIt([
    { b: { c: "c" } },
    { b: { b: { c: "c" } } }
  ])
]);

Type '{ b: { c: string; b?: undefined; }; } | { b: { b: { c: string; }; c?: undefined; }; }' is not assignable to type 'Config'.
  Type '{ b: { c: string; b?: undefined; }; }' is not assignable to type 'Config'.
    Property 'c' is missing in type '{ b: { c: string; b?: undefined; }; }' but required in type 'ConditionC'
      'c' is declared here.

🙂 Expected behavior

I expect that all of the cases above pass as they do in TS, since the types are correct.

Additional information about the issue

VSCode version 1.89.0
TS version 5.4.5

This is tested with checkJs set as true.

[Andarist]

It behaves the same way but you need to make sure to enable strictNullChecks for your JS file: TS Playground

I find it confusing that the TS playground for JS files uses strict: true, strictNullChecks: false. When I see strict: true set as the default I certainly don't go check all the other options to see if they are not opting out of strictness. This is done here: https://github.com/microsoft/TypeScript-Website/blob/eaa8205658445d4df6c0ca04e42fab1019f71df0/packages/sandbox/src/compilerOptions.ts#L21

[Arkellys]

Ah yes you're right, the issue only appears when strictNullCheck is false. Here is the updated playground. I wish I could use this as a workaround for this bug, but turning on strictNullCheck gives me even more errors for things I don't want. 🫤

[Andarist]

You can replace your nevers here with undefined and that should fix the problem without you having to turn on strictNullChecks: TS playground