Assigning array values by index treats Array<A> | Array<B> as Array<A | B>

TypeScript Version: 4.0.0-dev.201xxxxx reproducible starting from at least 3.3.3

Search Terms: discriminated unions, distributive

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.

function main() {
    var a: (number[] | string[])  = []; 
    a[0] = 3;
    a[1] = "xxx";
}

Expected behavior: error TS2322: Type ‘“xxx”’ is not assignable to type ‘number’.

Actual behavior: All is compiled without warnings or errors. Keep in mind that this works as expected:

var a: (number[] | string[])  = [3, "xxx"]; // Not OK as it should be 

Thus, depending on how exactly we’re initialising the array, behaviour differs. Playground Link: https://www.typescriptlang.org/play?ts=4.0.0-dev.20200508#code/C4TwDgpgBAggTnAcgVwLYGVhygXigOzQCMI4BtAXSgB8oBnLAS3wHNKBuAKE4DNl8AxsEYB7fFFQBDZgAoAlFADenKKqgA3SdkkAuKDMKoS5KrQZxmbCgtxQOUFWslkADFTwBmLmqjOAjO5QAEQAHmFBXAC+nEA

Related Issues:

[MicahZoltu]

I would personally expect TypeScript to error on me if I ever tried to assign a value to a union of arrays where the value being assigned was not compatible with all arrays in the union.

declare const apple: number[] | string[]
apple[0] = 5 // expect error
apple[0] = 'hello' // expect error
declare const banana: 'hello'[] | string[]
banana[0] = 'hello' // expect ok
banana[0] = 'goodbye' // expect error

[lazytype]

A similar concern arises with discriminated unions

declare const discriminatedUnion:
    | { type: 'string', value: string }
    | { type: 'number', value: number };

// This is not safe but there's no error
discriminatedUnion.type = 'number';