Regression: Partial properties became undefined instead of optional

[sveyret]

TypeScript Version: 3.9.0-dev.20200310

Search Terms:
Partial

Code

export interface Messages {
 [key: string]: string
 $: string
}
export type PartialMessages<T extends Messages> = Partial<T> & {
 $: string
}
export function createMessages<T extends Messages>(messages: T): T {
 return messages
}
export const en = createMessages({
 $: 'English',
 welcome: 'Welcome!',
}
export const fr_ca = createMessages<PartialMessages<typeof en>>({
 $: 'Français (Canada)',
})

Expected behavior:

Should compile. Messages in PartialMessages are either complying or absent.

• last tested working version: Typescript 3.7.5
• first tested failing version: Typescript 3.8.2

Actual behavior:

Fail in:

export const fr_ca = createMessages<PartialMessages<typeof en>>(

Consider that all messages are present, but may be undefined.

Type 'PartialMessages<{ $: string; welcome: string; }>' does not satisfy the constraint 'Messages'.
 Property 'welcome' is incompatible with index signature.
 Type 'string | undefined' is not assignable to type 'Message'.
 Type 'undefined' is not assignable to type 'Message'.

[RyanCavanaugh]

Anders Hejlsberg (@ahejlsberg) thoughts on this?

[MKRhere]

Partial<T> seems to make property values optional rather than the keys. Even though it seems like a semantic difference, a non-existent property cannot be accessed by messages.<prop_name>, while a property with a potentially non-existent value may be accessed to give type | undefined. Quick fix for now would be something like:

export type Messages = Partial<{
 [key: string]: string,
}> & {
 $: string
}

[ahejlsberg]

This is working as intended and is an effect of #35143 (which was in the 3.8 release). Consider:

// Compile with --strictNullChecks
declare let t: { [key: string]: string, $: string };
declare let s1: { welcome?: string } & { $: string };
declare let s2: { welcome?: string, $: string };

t = s1; // Error, but wasn't before 3.8
t = s2; // Error

Clearly the two assignments should be checked the same and they are now. Note that neither assignment is an error when --strictNullChecks is off.

[sveyret]

Thank you Anders Hejlsberg (@ahejlsberg). So, if I understand, the best way to write what I expect is how suggested by Muthu Kumar (@MKRhere)?
I suggest to add this behavior (and the solution) in the « breaking change » list of 3.8…

[ahejlsberg]

Stéphane Veyret (@sveyret) I would probably change the declaration of Messages to

export interface Messages {
 [key: string]: string | undefined;
 $: string;
}

[sveyret]

Yes, that's what I finally did. Thank you.
Shall I close the issue?