Object spread does not respect optional undefined union when strictOptionalProperties is on

[HerringtonDarkholme]

Bug Report

Another issue elated to #44421. This one seems to be more problematic due to runtime unsoundness.

🔎 Search Terms

strict optional properties, object spread, undefined

🕗 Version & Regression Information

Nightly, with strictOptionalProperties on

• I was unable to test this on prior versions because strictOptionalProperties is a new option

⏯ Playground Link

Playground link with relevant code

💻 Code

var a: {a: string} = {a: ''}
var b: {a?: string} = {}
var c: {a: string | undefined} = {a: undefined}
var d: {a?: string | undefined} = {a: undefined}

var a1 = {a: 123, ...a} // error: "a" specified more than once
var b1 = {a: 123, ...b} // {a: number|string}
var c1 = {a: 123, ...c} // expect error "a" specified more than once
var d1 = {a: 123, ...d} // expect {a: string|number|undefined}

d1.a.toString() // crash! should report error

🙁 Actual behavior

c1 created without duplicate property error and d1 is inferred as {a: string | number}

🙂 Expected behavior

TS should report error on c1 and d1 should be inferred as {a: string|number|undefined} to prevent runtime crash.