unknown && false is too loosely typed

[Diggsey]

Bug Report

The following code should type-check: https://www.typescriptlang.org/play?#code/FAFwngDgpgBAYgQwDYGcwwLwwHYFclIwA+Mu2AJlAGYCW2U5xMVyKsJeAtgEZQBOTAAzYmAIlFMAEgBUAsgBkAggQDCAewJQAxiBprswYFTI69IqmrUAKAB4AuUtgDW2NQHdsASgeJU6AN7AMMEwfFAguHwiNjAAZLHMrFDAAL7AQA

🔎 Search Terms

unknown falsy false and &&

🕗 Version & Regression Information

Please keep and fill in the line that best applies:

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about unknown values and &&

⏯ Playground Link

Playground link with relevant code

💻 Code

type Falsy = null | undefined | false | number | 0n | "" | HTMLAllCollection

function foo(x: unknown): Falsy {
    return x && false
}

🙁 Actual behavior

Fails to type-check

🙂 Expected behavior

Should type-check

This would be improved further if:

• NaN was its own type, so Falsy could be more specific than number. (Add a --strictNaNChecks option, and a NaN / integer / float type to avoid runtime NaN errors #28682)
• Falsy/Truthy were built in types.
• HTMLAllCollection variant could be omitted as a legacy value since document.all is deprecated and it's otherwise the case that objects are not falsy.