Source

File: wp-includes/class-wp-customize-setting.php

public function validate( $value ) {
		if ( is_wp_error( $value ) ) {
			return $value;
		}
		if ( is_null( $value ) ) {
			return new WP_Error( 'invalid_value', __( 'Invalid value.' ) );
		}

		$validity = new WP_Error();

		/**
		 * Validates a Customize setting value.
		 *
		 * Plugins should amend the `$validity` object via its `WP_Error::add()` method.
		 *
		 * The dynamic portion of the hook name, `$this->ID`, refers to the setting ID.
		 *
		 * @since 4.6.0
		 *
		 * @param WP_Error             $validity Filtered from `true` to `WP_Error` when invalid.
		 * @param mixed                $value    Value of the setting.
		 * @param WP_Customize_Setting $this     WP_Customize_Setting instance.
		 */
		$validity = apply_filters( "customize_validate_{$this->id}", $validity, $value, $this );

		if ( is_wp_error( $validity ) && ! $validity->has_errors() ) {
			$validity = true;
		}
		return $validity;
	}