01_vec_varray.hack

use namespace HH\Lib\C;

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

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

function takes_vec(?vec<string> $v): void {}

02_run.log

$ hhvm vec_varray.hack                             

Fatal error: Uncaught TypeError: Argument 1 to takes_vec() must be of type ?HH\vec, array given in .../vec_varray.hack:21
Stack trace:
#0 .../vec_varray.hack(18): takes_vec()
#1 .../vec_varray.hack(6): takes_varray()
#2 (): example()
#3 {main}