@ChrisW67 thank you.

Which one do you mean ?

Thanks guys for replies, generally my app isn't well designed because it's for education purpose. Anyway thank you very much.

@ealione
The good thing is that typically users do not notice. The problem is popping up when trying to compile with older compilers or you remember that such stuff was not possible in the past.
As @mcosta suggests there is probably a recommendation out. On the other compiler versions are changing too rapidly and also the versions of recommendations.