Class union type refinement takes too long.

[Zmetser]

Refining an union of class that share a common base takes a lot longer than it should. Especially, when the refining code has conditions like this instance instanceof AA || instance instanceof AB.

Here's a code example that will still run in the online sandbox:

// @flow
class Base {}

class A extends Base {}
class AA extends Base {}
class AB extends Base {}
class B extends Base {}

type ClassUnion = A | AA | AB | B;

function test(instance: ClassUnion): ?string {
    if (instance instanceof A) return 'A'
    else if (
        instance instanceof AA ||
        instance instanceof AB
    ) return 'ASomething'
    else if (instance instanceof B) return 'B'
    else unexpectedCase(instance);
}

function unexpectedCase(impossible: empty): void {
    throw new Error(`Unexpected case ${impossible}`);
}

Here's one that takes 10s to run, freezes the online sandbox and always does a merge on flow check regardless of what changed. (looks like the real deciding factor is not the number of is statements but the number of conditions in an if statement.)

[Zmetser]

Union refinement is still slow as of v0.86.0.

Try it here

The workaround is to have all check in a new else if condition