Bug: Intellisense shows misleading error when parameter is expected to be a union of an object and a string

[devanshj]

Bug Report

🔎 Search Terms

Intellisense, incorrect function parameter

🕗 Version & Regression Information

Tried with v4.3.5

⏯ Playground Link

Playground

💻 Code

declare const send:
  (event: { type: "A", foo: string } | { type: "B" } | "B") => void

declare const sendOnlyObject:
  (event: { type: "A", foo: string } | { type: "B" }) => void


send({ type: "A" })
// Type '"A"' is not assignable to type '"B"'.(2322)

sendOnlyObject({ type: "A" })
// Argument of type '{ type: "A"; }' is not assignable to parameter of type '{ type: "A"; foo: string; } | { type: "B"; }'.
//   Property 'foo' is missing in type '{ type: "A"; }' but required in type '{ type: "A"; foo: string; }'.(2345)

🙁 Actual behavior

The error for send({ type: "A" }) is Type '"A"' is not assignable to type '"B"'

🙂 Expected behavior

The error should be something like the one for sendOnlyObject or at least not misleading that the property type has to be "B" that's what is getting suggested here.

[DanielRosenwasser]

It's probably the case that we bail early in findMatchingDiscriminantType because of the primitive. If it's not too hard and it's not too expensive, the fix is likely to create a union of only object types, match on the discriminants of the union, and elaborate on that type.

[MonstraG]

I'm not sure if it's related, but I've just hit TS2345 on this:

const func = (foo: boolean | "bar") => {}
func(1 != 2 && "foo")
     ^^^^^^^^^^^^^^^
TS2345: Argument of type '"" | "foo"' is not assignable to parameter of type 'boolean | "bar"'.   Type '""' is not assignable to type 'boolean | "bar"

(this contrived example also gives TS2367 but we don't care about that)

It looks like for some reason, if I | with a string, first false is resolved as "" instead

[ldwonday]

Is the problem fix in v4.5.2?

[MonstraG]

Neither OP nor mine is fixed on 4.5.2