code: private function expect($target, Constraint $constraint): self {
      assertThat($target, $this->hasFlag('negate') ? logicalNot($constraint) : $constraint, $this->message);
      return $this;
    }
docs: Asserts that the specified target matches the specified constraint.
  @param mixed $target The target to check.
  @param Constraint $constraint The constraint to match.
  @return self This instance.