Understanding result of indexed access types and extends

Issue

How does the indexed access type work when combined with extends keyword?

I can't explain why is result of following false:

type Test<T extends readonly any[]> =  T[number] extends true ? true: false
type R = Test<[true,true,false]>

Result:

false

Solution

Let's go through this step by step.

T[number] will evaluate to true | true | false if T is [true,true,false]. TypeScript conveniently collapses this union down to just boolean.

boolean does not extend true, so the conditional evaluates to the false branch returning false.

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You may have assumed that the union would be distributed over the conditional. But T[number] is not a naked generic type. No distribution takes place.

Answered By - Tobias S.