Falsy checks for empty vecs break type checker soundness

[ryangreenberg]

Describe the bug An empty vec[] does not pass a truthy check but it still a vec. The typechecker incorrectly infers that values in the other branch of this check are not vecs.

Standalone code, or other way to reproduce the problem

The following code typechecks but results in a fatal error at runtime under hhvm 4.108

empty_falsey.hack

use namespace HH\Lib\C;

<<__EntryPoint>>
function example(): void {
        takes_vec(vec[""]); // works for non-empty vec
        takes_vec(vec[]); // runtime exception for empty vec
}

function takes_vec(?vec<string> $v): void {
        // This truthy check does not pass for an empty vec, but the typechecker doesn't
        // track the fact that empty vec miss this branch and are not converted to vecs
        //
        // If you replace the if ($v) with the next line, the typechecker can see the problem
        // if ($v is nonnull && !C\is_empty($v)) {
        if ($v) {
                $v = 5;
        }

    // $v is inferred to be ?int but is `?(int | vec<string>)`
    takes_int($v);
}

function takes_int(?int $i): void {}

$ hh_client
No errors!

$ hhvm empty_falsey.hack 

Fatal error: Uncaught TypeError: Argument 1 to takes_int() must be of type ?int, HH\vec given in /Users/greenberg/code/hhvm_repro/empty_falsey.hack:23
Stack trace:
#0 /Users/greenberg/code/hhvm_repro/empty_falsey.hack(20): takes_int()
#1 /Users/greenberg/code/hhvm_repro/empty_falsey.hack(6): takes_vec()
#2 (): example()
#3 {main}

Expected behavior

Typechecker error on line 20 (takes_int($v);) saying "Expected ?int, got vec

Actual behavior

(no error)

Environment

• Operating system: MacOS Big Sur 11.2.3
• Installation method: brew install hhvm/hhvm/hhvm-4.108

$ hhvm --version
HipHop VM 4.108.0 (rel)
Compiler: 1620152370_N
Repo schema: 09d2af8dd22201dd8d48e5dcfcaed281ff9422c7

$ hh_client --version
hackc-893a8c56ba3cbd052c6cff6e404215635966a29b-4.108.0

[fredemmott]

Thanks - also repros on our internal builds which is helpful. Filed FB T91160732 for this