The Visual Studio Qt plugin manages the project file (VS version of a Makefile), provides some wizards for adding new Qt-based classes to projects, can import and export .pro files, adds build and link rules to run uic and moc when needed, and so forth. For ui files, opening them for edit will run Qt Designer by default. It would be nearly impossible to use Visual Studio productively on Qt projects without it.

It sounds to me like the OP has broken the UI file. I'm not sure how that is possible if the only changes that were made were done through Qt Designer unless something happened when saving the file to cause corruption.

It is also possible that if a new widget was added to the .ui file, the appropriate #include files might not have been added to the C++ class source files. This doesn't happen automatically. A "C++ does not support default int" error usually arises when the return type for a method or an argument type for a method has not been specified (or can't be determined from the types in scope during the compilation).

I'd suggest deleting the .ui file and using Qt Designer in stand-alone mode to re-create a new file with the same name as the old one. If the old file can still be opened in Qt Designer, then rename it, open it, create a new widget / dialog, and drag and drop the old widgets and layouts into the new one.