Type guard not working in nested conditional types

[07akioni]

Bug Report

🔎 Search Terms

type guard, nested conditional types

🕗 Version & Regression Information

• This is a crash
• This changed between versions ______ and _______
• ✅ This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.4.2#code/C4TwDgpgBAggJnACgJwPZgCqoPICMBWEAxsADx75QQAewEAdnAM5SoHHAA0UK6VtDZlCbBkAS3oBzAHxQAvDzRh+dRiwDWEEKgBmUCgCgoUAPz6CAbV5gAuisEsArvXX1UAd3oWbR46agWAHTBFFZKNj5+UABcUPQQAG4QyL6x8UnIQA

Playground link with relevant code

💻 Code

type AddPropToObject<Obj extends object, Prop extends string> = Prop extends keyof Obj
  ? Obj[Prop] extends unknown[]
    ? [...Obj[Prop]]
    : never
  : never

🙁 Actual behavior

Obj[Prop] is not viewed as an array.

🙂 Expected behavior

Obj[Prop] should be viewed as an array.

[andrewbranch]

Looks like we’re failing to create/use a substitution type here on the indexed access type. It works if you move the inner conditional type into its own type alias.

[andrewbranch]

Note to self or other future fixer: use #46145 as another test case