Creating Blank Qt Widgets Application demonstrates "Allocation of incomplete type" Error/Warnings

Qt Open Source. Deployed MinGW Qt 5.15.2 and MinGW Qt 6.3.1 (issue same in all versions) only. Full deploys of these kits for MinGW only.

Select File/New Project/Application Qt/Qt Widgets Application

Click Choose

(Project Location and Name is irrelevant; using the default "untitled")

Click Next

Define Build System: qmake (note that cmake is also deployed)

Click Next

(Details all default)

Click Next

(Translation - Defaults)

Click Next

Kits: Select Desktop Qt 5.15.2 MinGW 64 Bit OR Desktop Qt 6.3.1 MinGW 64 Bit

Click Next

(Summary - Accept Defaults)

Click Finish

(wait for all components to generate and for systems to update etc.)

• • • Comment: We have a blank, starting template. ***

Select Build / Run qmake (or in fact run etc).

All compiles BUT code demonstrates issues as shown in attachment.

Summarised: 

All issues in file MainWindow.cpp (auto generated)

(a) C:\Users\stepheni.HOME\Documents\Qt Projects\untitled\mainwindow.cpp:2: error: 'ui_mainwindow.h' file not found

(b) C:\Users\stepheni.HOME\Documents\Qt Projects\untitled\mainwindow.cpp:6: error: Allocation of incomplete type 'Ui::MainWindow'

(c) C:\Users\stepheni.HOME\Documents\Qt Projects\untitled\mainwindow.cpp:8: error: Member access into incomplete type 'Ui::MainWindow'

(d) C:\Users\stepheni.HOME\Documents\Qt Projects\untitled\mainwindow.cpp:13: warning: Deleting pointer to incomplete type 'Ui::MainWindow' may cause undefined behavior

(See proof attached)

Note that Default templates should just work. These throw issues.

Note that code compiles and executes.