Source

File: wp-includes/Requests/IRI.php

public function is_valid() {
		$isauthority = $this->iuserinfo !== null || $this->ihost !== null || $this->port !== null;
		if ($this->ipath !== '' &&
			(
				$isauthority && $this->ipath[0] !== '/' ||
				(
					$this->scheme === null &&
					!$isauthority &&
					strpos($this->ipath, ':') !== false &&
					(strpos($this->ipath, '/') === false ? true : strpos($this->ipath, ':') < strpos($this->ipath, '/'))
				)
			)
		) {
			return false;
		}

		return true;
	}