Create real darrays for shapes.

refs facebook/hhvm#8343

Not sure if `as` requiring darrays is a bug or not, but this is also
correct, so let's do it.

Author: fredemmott

src/TypeSpec/__Private/ShapeSpec.php

@@ -120,7 +120,8 @@ private static function dictToShapeUNSAFE(
       /* HH_IGNORE_ERROR[4110] */
       return $shape;
     }
-    /* HH_IGNORE_ERROR[4007] */
-    return (array)$shape;
+    /* HH_FIXME[2049] */
+    /* HH_FIXME[4107] */
+    return \darray($shape);
   }
 }

tests/TypeStructureTest.php

@@ -216,6 +216,17 @@ public function testValidType<T>(TypeStructure<T> $ts, T $input): void {
       ->toBeSame($input);
   }
 
+  public function testAssertedShapeAsShape(): void {
+    // most of the runtime is fine with actual PHP arrays, but we need a real
+    // darray for the `as` operator
+    $x = shape('someString' => 'foo');
+    $y = TypeAssert\matches_type_structure(
+      type_structure(TypeConstants::class, 'TShapeWithOneField'),
+      $x,
+    );
+    $z = $y as TypeConstants::TShapeWithOneField;
+  }
+
   public function getExampleInvalidTypes(
   ): array<string, (mixed, mixed, vec<string>)> {
     $examples = [