Fix type error in Vec\fill

Summary:
array_fill can return false if $size<0. This results in a cast from bool to vec, leading to a InvalidOperationException with a confusing message.

Instead of invoking array_fill with invalid arguments, throw an InvariantException.

Reviewed By: kmeht

Differential Revision: D5652216

fbshipit-source-id: 16995500392e07f74601cd5e99c8ca7763268055

src/vec/transform.php

@@ -40,6 +40,7 @@ function fill<Tv>(
   int $size,
   Tv $value,
 ): vec<Tv> {
+  invariant($size >= 0, 'Expected non-negative fill size, got %d.', $size);
   return vec(\array_fill(0, $size, $value));
 }
 

tests/vec/VecTransformTest.php

@@ -11,6 +11,7 @@
 
 use namespace HH\Lib\Vec;
 use function \Facebook\FBExpect\expect;
+// @oss-disable: use InvariantViolationException as InvariantException;
 
 /**
  * @emails oncall+hack_prod_infra
@@ -87,6 +88,10 @@ public function testFill<Tv>(
     expect(Vec\fill($size, $value))->toBeSame($expected);
   }
 
+  public function testFillExceptions(): void {
+    expect(() ==> Vec\fill(-1, true))->toThrow(InvariantException::class);
+  }
+
   public static function provideTestFlatten(): array<mixed> {
     return array(
       tuple(