Regression: Indexed Access of Partial Mapped Type with Union from Generic Function is missing `undefined`

[3x071c]

🔎 Search Terms

generic function, partial, optional, mapped type, indexed access, undefined, bug, regression

🕗 Version & Regression Information

• This is a bug
• This changed between versions 4.5.5 and 4.6.2
• This is the behavior in every version I tried (all playground versions)

⏯ Playground Link

https://www.typescriptlang.org/play?target=99&jsx=0&pretty=true&ts=4.6.2#code/C4TwDgpgBA8gRgKygXigbwFBW1AhgLigEYAaLHOQgJgwF8BuDDAYwHsA7AZ2CgFtcwkACaFMOKAG0A0lACW7KAGsIIVgDMoABVwAnYLNwAbADzwEAPgC6hM9Mt0U6WkzZceOiJ1aGAbhACyAsKOxjIQAB7AEOxCnEoq6lCgkIn8ghBC5gAUyiCEUgCUKOZ8QRkSuZYAdMCsAMrAOvIA5lkFjAD0HVAAegC0fVAAojo6rDpQEZDMUUJQWQAGacIVKpYLcnFgrJycsnCGIFAArjEQavIZBRhAA

💻 Code

type Obj = {
    a: 1,
    b: 2
};

const mapped: {
    [K in keyof Partial<Obj>]: Obj[K]
} = {}

const resolveMapped = <K extends keyof typeof mapped>(key: K) => mapped[key].toString();
// ^-- Error expected (`mapped[key]` is possibly undefined)

🙁 Actual behavior

TypeScript does not recognize mapper[key] might be undefined and emits broken code without errors

🙂 Expected behavior

Correct type inference from pre-v4.6.x

Additional information about the issue

No response

[RyanCavanaugh]

Isn't the bug earlier? The type { [K in keyof Partial<Obj>]: Obj[K] } should be { a: number, b: number } and the initialization from { } should fail.

[3x071c]

@RyanCavanaugh The mapped definition seems to work as intended ({ a?: number, b?: number }), as it does in all minor TypeScript versions since 3.3.3 (from the playground). Kinda surprised if that actually isn't expected behavior.

[RyanCavanaugh]

Bisects to #47109

[jcalz]

From here