Narrow subtype-reduction-prone unions to their narrowest constituent

Bug Report

Type narrowing doesn’t seem to take into account an assignment I made to ensure the variable has the right type.

Maybe related? https://github.com/microsoft/TypeScript/issues/43584#issuecomment-816932485

🔎 Search Terms

type narrowing assignment

🕗 Version & Regression Information

4.5.4

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about “type narrowing”

⏯ Playground Link

Playground Link

💻 Code

declare let a: object | any[] | undefined

if (a === undefined) {
    a = [] // I expect this line to have TS understand that a is any[] from now on
} else if (!Array.isArray(a)) {
    throw new Error()
}
[...a] // complains here because a is object | any[]

🙁 Actual behavior

a has type object | any[]

🙂 Expected behavior

a to have type any[]

[RyanCavanaugh]

You could write

declare let a: { push?: never } | any[] | undefined

if (a === undefined) {
    a = [] // I expect this line to have TS understand that a is any[] from now on
} else if (!Array.isArray(a)) {
    throw new Error()
}

Subtype-reducing unions aren’t great but it seems like we could fix the narrowing-on-assignment logic to do a subtype reduction on any resulting union, since that’s assured to be sound.

[fatcerberus]

This is expected; arrays are assignable to object.