Cannot create a Partial<Generic> type in a one-liner

[gewoonwoutje]

TypeScript Version: 3.4.5; 3.7.0-dev.20190829

Search Terms:

Partial, Generic, inconsistent,

Code

interface MyMap {
	field: string;
}

function someGenericFunction<T extends MyMap>(input: T) {
	let y: Partial<T>;

	// falsely not allowed: 
	// ts(2322): Type '{ field: ''; }'is not assignable to type 'Partial<T>'. 
	y = { field: '', };

	// similar, but allowed
	y = {};
	y.field = '';

	// correctly not allowed
	y.anything = ''; // anything is not defined in MyMap type
	y = {
		field: '',
		anything: 123, // anything is not defined in MyMap type
	}
}

Expected behavior:
Should be able to create the generic in a one-liner .

Actual behavior:

Can be done in two lines, not in one line.

Playground Link:

http://www.typescriptlang.org/play/#code/JYOwLgpgTgZghgYwgAgLIE9VwA7IN4CwAUAJAzAQA2AJgFzIDOYUoA5gNzEC+xxMAriARhgAexCNRAWwgBxCCGjAEAMUHCxIADwAVZBAAekENQZpMOAHwAKUNn5h6OgJT5iJShDDJ09AApwUCJwlLqWnETuAPRRyPCUDFToyCCi3iGUogDuEHTI0bFgDNYATADMJSXOTujYKADkeHEUNPT19ezIXPXAZqnpDAzArCBwAEaeyGCiU7UNAUHAIWH1AHT5pMkAvPjNVHntADRdEQWMwFL8lIHHYw7IGdm57tv4XBEk6Kvk+8g77adIqQYsgEKIoFAIMJKMl+g9KJkctQXqs4CB0GAABZsP7IAHIEE-Gh4tEY7EgVj1ZDUUQQPppfQGXreUCzOrmLDYF64wikMgtA71Q7uEikrFsegARnKx0JApJ6PFFKpNLpKQZhmZyFZYDmHJw7h4RCNQA

Related Issues:
#28470

[jcalz]

Duplicate of #13442

Note that T may be resolved with a type like

interface SomeoneElsesMap extends MyMap {
  field: "It's *my* map now";
}

So you can't safely assign {field: ""} to Partial<T>. For that matter, you can't safely do the assignment y.field = "", but for whatever reason that's allowed by the compiler.

[gewoonwoutje]

Okay, I see I did not do enough search in the issue history. This issue has a lot of history being closed as duplicate. In fact, item #23015 seems to be exactly the same.

It's still a bug though, as it does not work like expected. Or it should accept both cases, or neither. But I understand that it would be a very breaking change in both cases.