type cast in tuples

TypeScript Version: 3.1.6

Code

		const a1:[number, number, boolean] = [1, 0, false];
		//... code
		if (a1[2]) {
			a1[2] = false; //error TS2322
		}

Expected behavior:
no error (run in TypeScript 2.9.2)

Actual behavior:
TS2322: Type 'false' is not assignable to type 'true'.

why is the type cast to true ?

if (a1[2]) checks for truthiness and thus narrows the type of a[2] from boolean to true.

first of all, this has nothing to do with type casting. this is an issue with narrowing.

This is fixed by #27574 which is part of v3.2.0-rc (and probably this week in v3.2.1)

This issue has been marked as a duplicate and has seen no activity in the last day. It has been closed automatic house-keeping purposes.

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