Consider unknown values true-ish in RouterInterface CustomizeDi… (#5178)

Signed-off-by: Modular Magician <[email protected]>

google/resource_compute_router_interface.go

@@ -296,8 +296,15 @@ func routerInterfaceDiffOneOfCheck(d *schema.ResourceDiff, meta interface{}) err
 	_, ipOk := d.GetOk("ip_range")
 	_, vpnOk := d.GetOk("vpn_tunnel")
 	_, icOk := d.GetOk("interconnect_attachment")
-	if !(ipOk || vpnOk || icOk) {
+	// When unset, these values are all known. However, if the value is an
+	// interpolation to a resource that hasn't been created, the value is
+	// unknown and d.GetOk will return false. For each value, if that value is
+	// unknown, consider it true-ish.
+	if !((ipOk || !d.NewValueKnown("ip_range")) ||
+		(vpnOk || !d.NewValueKnown("vpn_tunnel")) ||
+		(icOk || !d.NewValueKnown("interconnect_attachment"))) {
 		return fmt.Errorf("Each interface requires one linked resource or an ip range, or both.")
 	}
+
 	return nil
 }