Incorrect inferring of return type

Version: 3.4.5

declare function abc<T>(q: T | [{id: 'Id'}]): T 
var x = abc([{id: 'Id'}]).map(v => v.id /* 'Id' */) // string[] 

var y = abc([{id: 'Id'}]).map(v => v.id as 'Id') // 'Id'[]

Related: #30680

It is due to widening.
If you want to narrow type you have to specify explicitly as you did or use some tricks.

effort level matching answer: don’t see a bug here, try SO

I'd refer to #241 for explaining away a return type being wider than expected

Something strange happens here

declare function abc<T>(q: T | {id: 'Id'}[]): T 
var u = abc([{id: 'Id'}]) // -> {id: 'Id'}[]
var k:{id: 'Id'}[] = u; // no problem
var l:typeof u = k; // no problem
var e:{id: 'Id'}[] = {} as {id: string}[]; // error (correct)
var p = k.map(v => v.id); // -> 'Id'[]
var t = l.map(v => v.id); // -> string[] (incorrect)

k's literal type isn't "fresh" since it came from a type position instead of an expression, so isn't widened. (While since l is typeof u it's still fresh)

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

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Incorrect inferring of return type #31254

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Incorrect inferring of return type #31254

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NN--- commented on May 4, 2019

jcalz commented on May 5, 2019

weswigham commented on May 8, 2019

cevek commented on May 9, 2019

weswigham commented on May 9, 2019

typescript-bot commented on May 11, 2019

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