As I wrote earlier I’ve switched back to Visual Studio Code on my Linux Mint 21.1 system for C++ development. In order to support Qt 6 C++ development, I needed to modify the C++ plugin’s include path to point to specific Qt 6 include files. The easiest way to do this is (and this assumes you have the Microsoft C/C++ extension installed to begin with):

1. Select Extensions in Visual Studio Code on the far left.
2. On the C/C++ Extension left-click the gear.
3. On the menu that appears, select Extension Settings.
4. In the Settings screen that opens, scroll down to C_Cpp > Default: Include Path.
5. Click the blue Add Item button.
6. In the input text box that appears, type in the full path to the Qt include files. Note in my example I have two paths to the Qt 6 include files.

Once you’ve updated the include paths just close the Settings tab. If you don’t have those additional paths in the Microsoft C/C++ plugin, then any and all Qt include files will have the dreaded Red Squiggly Lines underlining all those include files it can’t find.