Spreading multiple objects into one doesn't type check correctly

[mhanuszh]

Bug Report

🔎 Search Terms

spread operator
object types

🕗 Version & Regression Information

Version 4.8.4
Version 5.0.0-dev.20221103

⏯ Playground Link

Playground link with relevant code

💻 Code

function processStrings(input: {[index: string]: string}) {
	// Expecting an object with values of string type (with `length` property)
	for (var key in Object.keys(input)) console.log(input[key].length);
}

function test(parameters: {[index: string]: string | number}) {
	var myStuff = {...{asd: "qwe"}, ...parameters};
	// No errors here, while `myStuff` clearly can contain numbers
	processStrings(myStuff)


	var myStuff2 = {...parameters};
	// When leaving out the other object, the error is now correctly determined
	processStrings(myStuff2)
}

// Providing an object with value of number type
test({"key": 123});

🙁 Actual behavior

The myStuff variable doesn't receive the predictable {[index: string]: string | number} type, therefore the script crashes at runtime

🙂 Expected behavior

The myStuff variable should receive the predictable {[index: string]: string | number} type, therefore the IDE or the compiler should show errors about this

[fatcerberus]

See #50559 and, in particular, #50185 (comment).

Spread is currently modeled using intersection types, which sometimes produces the wrong type. To model this correctly TS would need a dedicated type operator for spread.

[mhanuszh]

See #50559 and, in particular, #50185 (comment).

Spread is currently modeled using intersection types, which sometimes produces the wrong type. To model this correctly TS would need a dedicated type operator for spread.

So, this is a known TS limitation. Thank you for the clarification!

[fatcerberus]

Yeah, specifically the limitation is that TS models { ...t, ...u } as T & U, but this is wrong when t and u have overlapping properties (a string index signature is considered to overlap with everything for this purpose).

The issue you want to follow is #10727.