TS should not allow assigning value with undefined union to non-nullable property when strictOptionalProperties

[HerringtonDarkholme]

Bug Report

First, congrats to the new compiler switch!
I don't know if this is intentional since strict optional property semantics seems tricky. Related to #44421.

🔎 Search Terms

strict optional properties, assignability

🕗 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

declare var a: {[x: string]: number | string }
declare var b: {a: number, b: string}
declare var c: {a: number, b?: string}
declare var d: {a: number, b: string | undefined }
declare var e: {a: number, b?: string | undefined }

a = b // ok
a = c // ok 
a = d // error
a = e // expect error

🙁 Actual behavior

TS allow a = e where the property b might be undefined.

🙂 Expected behavior

TS should report error because of consistency?

[RyanCavanaugh]

@ahejlsberg I'm inclined to agree but let's discuss today