null coalescing operator doesn't narrow types in some complex circumstances involving generics

[Zachiah]

Bug Report

🔎 Search Terms

Null Coalescing operator:
I found this issue which is very similar, but I don't think it's the same. I mean it seems to me that if the compiler can correctly narrow the type with the ternary it should do the same for the null coalescing

🕗 Version & Regression Information

• I have determined that in v4.2.3 all 3 examples from my code fail, and that in v4.3.5 only the last fails. I think none of them should fail, but I'm not positive on that, only positive that they all should behave the same

⏯ Playground Link

Playground Link

💻 Code

type TagName = "div" | "span" | "li" | "ul" | "ol";

export type ForwardingProps<T extends TagName, Default extends TagName> = Default extends T ? {
  as?: T;
  ref: HTMLElementTagNameMap[T]
} : {
  as: T;
  ref: HTMLElementTagNameMap[T]
}

const fn = <T extends TagName = "li">(p: ForwardingProps<T, "li">) => { }


const fn2 = <T extends TagName = "span">(p: ForwardingProps<T, "span">) => {
  const {as = "span"} = p;
  const as2 = p.as ? p.as : "span";
  const as3 = p.as ?? "span";

  fn({
    as: as,
    ref: p.ref
  });

  fn({
    as: as2,
    ref: p.ref
  })

  fn({
    as: as3,
    ref: p.ref
  })
}

🙁 Actual behavior

Typescript happily understands the first 2 attempts at removing null from the possible types (object destructuring defaults and the ternary operator) but complains when the null coalescing operator is used.

🙂 Expected behavior

The null coalescing operator should behave exactly the same as the equivalent ternary or object destructuring expression