[Typechecker | Regression] Intersection with enum types disables switch exhaustion check

[lexidor]

HHVM Version

HipHop VM 4.23.0-dev (rel)
Compiler: 1568605624_509374463
Repo schema: d3aeac4022c6a43fc3660e7d8141465a7fbb14c2
hackc-9f58f3799749c8d2680054e2c26f95325770390d-4.23.0-dev

Operating System and Version

Ubuntu 18.04

Standalone code, or other way to reproduce the problem

enum NUMBERS: string {
    ONE = 'ONE';
    TWO = 'TWO';
}

<<__EntryPoint>>
function snippet(): void {
    $one = 'ONE';
    $one as NUMBERS;
    switch ($one) {
        case NUMBERS::ONE:
            ;
    }

    $two = 'TWO';
    $two = NUMBERS::assert($two);
    switch ($two) {
        case NUMBERS::ONE:
            ;
    }
}

Actual result

hh_client 
Typing[4019] Switch statement nonexhaustive; the following cases are missing: TWO
   --> src/file.hack
  6 | enum NUMBERS: string {
    |      ^^^^^^^ Enum declared here
 22 |     switch ($two) {
    |             ^^^^

1 error found.

Expected result

Check about $one too.

[lexidor]

Edited repro, because empty switch{} statements are a parse error in newer hhvm versions.

[fredemmott]

@vsiles @CatherineGasnier 53a12e2 from @sarahhenkens is a suggested fix for this which appears to work, but looks like it's essentially a revert 7824d82 - do you have context to share on that or an alternative approach?

[CatherineGasnier]

I've commented on 53a12e2