Having `noImplicitAny` Not Enabled Impacts Type Inference

[joealden]

Bug Report

🔎 Search Terms

array union types type inference noImplicitAny disabled

🕗 Version & Regression Information

This changed between versions 4.2.3 and 4.3.5, as before 4.3.5, type inference for the 2nd case inferred any regardless of if noImplicitAny was enabled. After 4.3.5, the expected inference happens when noImplicitAny is enabled, but behaves as it did before 4.3.5 when noImplicitAny is not enabled.

⏯ Playground Link

Playground link with relevant code

💻 Code

// `item` and `key` are both typed as expected (`string | number` and `number` respectively)
const array1: Array<string | number> = [];
const mapped1 = array1.map((item, key) => [item, key])

// `item` and `key` are both typed as `any`
const array2: Array<string> | Array<number> = [];
const mapped2 = array2.map((item, key) => [item, key])

🙁 Actual behavior

When noImplicitAny is not enabled in the TS config, inferred types are impacted (inferred as any in the above case).

🙂 Expected behavior

noImplicitAny shouldn't impact inferred types, or if this is expected behaviour as having this option enabled allows for better heuristics to work, then I think having it documented that this settings isn't essentially just a "lint rule" might be a good idea.

[DanielRosenwasser]

For anyone else checking, it has nothing to do with the initializer.

Playground link with noImplicitAny disabled

function foo1(fnArray1: Array<string | number>) {
    // `item` and `key` are both typed as expected (`string | number` and `number`)
    const mapped1 = fnArray1.map((item, key) => [item, key])
}

function foo2(fnArray2: Array<string> | Array<number>) {
    // `item` and `key` are both typed as `any`
    const mapped2 = fnArray2.map((item, key) => [item, key])
}

[ahejlsberg]

This is apparently by design. See #42620. I presume code existed before that PR that depends on parameters being assigned type any when -noImplicitAny is disabled.

[DanielRosenwasser]

Thanks for digging that up, totally forgot about it!

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.