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:

Start with the self-contained TypeScript reproducer and run it through the command-line tsc, comparing indexed assignments with array-literal initialization. The issue names no source files or tests, so trace the checker path for indexed assignment on a union of array types. Done means the reproduced assignment reports the expected type error without regressing the related initialization behavior.