Generic conditional type narrowing doesn't cascade as expected

[milohansen]

Bug Report

I've tried my best to search for related issues and read the documentation thoroughly but failed to find any useful information. I'm sorry if this bug turns out to be duplicate or even not a bug at all.

🔎 Search Terms

conditional type if undefined

🕗 Version & Regression Information

All versions available in the playground (http://www.typescriptlang.org/play)

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Generics and Type Guards.

⏯ Playground Link

Playground link

💻 Code

type Foo<T extends number | string> = {
  value: T;
}

type Narrow<T extends number | string | undefined> = T extends undefined ? never : Foo<T>;
//                                                                                     ^
// ERROR: Type 'string | number | undefined' is not assignable to type 'string | number'.

🙁 Actual behavior

In the above example, T throws an error when passed to Foo because the compiler believes T could be undefined.

🙂 Expected behavior

The condition has checked if T extends undefined so I would expect T's type to be narrowed from string | number | undefined to string | number on the right side of the ternary operator.

[RyanCavanaugh]

I feel like there might be a theoretical justification for why we don't do this, but I can't reconstruct it at the moment

[jack-williams]

This is related, but I'm not sure if it is out-of-date: #29188 (comment)

FWIW, this works

type Narrow<T extends number | string | undefined> = T extends number | string  ? Foo<T> : never;