Inferred array type allows unsound covariance?

[jtbandes]

The following code should raise an error, but doesn't:

Try Flow

const a = [1];

function foo(arr: (number|string)[]) {
    arr.push("x");
}
foo(a);

function bar(arr: number[]) { }
bar(a);

This problem seems to be specific to arrays. If I replace all T[] with {x: T}, then I get the expected behavior.

Flow version: v0.146.0

Expected behavior

Either an error on foo(a), or an error on bar(a). If a is inferred to be number[], then foo(a) should be an error. If it's inferred to be (number|string)[], then bar(a) should be an error.

Actual behavior

When the type of the array is left off (inferred), it seems to produce unsound results. When the type is explicitly specified I get the expected results.

[mischnic]

It works as expected though when declaring the variable type explicitly.

const a = [1];
const b: number[] = [1];

function foo(arr: (number|string)[]) {
    arr.push("x");
}

foo(a); // no error
foo(b); // error

[jtbandes]

Yes, I guess it’s a bug with the inference behavior.

[noppa]

Seems like the same thing as in #8108 and #8593

[jtbandes]

I agree—thanks for linking these. I think my repro case is a bit simpler since only primitive types are used 🙂

[SamChou19815]

Fixed in 0.278.0