Literal strings with generics are inconsistant when strictNullChecks is false

[sebastien-de-saint-florent]

Bug Report

🔎 Search Terms

• string literal
• strictNullChecks
• keyof

🕗 Version & Regression Information

It works properly on v4.7.4 but is failing on v4.8.2 and with the nightly build.
It also works normally when strictNullChecks is true.

⏯ Playground Link

Playground link with relevant code

💻 Code

type Model = { s: string; b: boolean }
let pick: <Keys extends keyof Model>(properties: readonly Keys[]) => Pick<Model, Keys>
let transform: <T>(obj: T) => T

const result1 = transform(pick(["s"]))

const intermediate = pick(["s"])
const result2 = transform(intermediate)

🙁 Actual behavior

result1 has a type of Pick<Model, keyof Model> and result2 as a type of Pick<Model, "s">

🙂 Expected behavior

result1 and result2 should have the same type Pick<Model, "s">.

[ahejlsberg]

@typescript-bot bisect good v4.7.4 bad v4.8.2

[ahejlsberg]

Was broken by #49086. Manually bisected to a2b785b.

[andrewbranch]

Adding a T extends {} constraint to transform makes it repro in --strictNullChecks too.

[andrewbranch]

I added a fix to #50759, which needs a review. It’s the same fix for #50635, but from one additional call site.