Qt5 QtCreator Error on Linux: stddef.h: No such file or directory – Code model could not parse an included file

The following is an error that has shown itself every time I have installed the Qt5 framework and the QtCreator development environment on a Linux based machine. It never mattered which flavor of Linux; QtCreator always showed this error.

Warning: The code model could not parse an included file, which might lead to incorrect code completion and highlighting, for example. 

fatal error: 'stddef.h' file not found 
note: in file included from /home/rlo/Code/C++/WorkTracker2/WorkTracker2Shared/src/data/taskrepository.h:1: 
note: in file included from /home/rlo/Code/C++/WorkTracker2/WorkTracker2Shared/src/data/taskrepository.h:3: 
note: in file included from /usr/include/c++/9/optional:38: 
note: in file included from /usr/include/c++/9/stdexcept:38: 
note: in file included from /usr/include/c++/9/exception:143: 
note: in file included from /usr/include/c++/9/bits/exception_ptr.h:38: 

Although that message never caused any issues compiling the code, I found it rather annoying, and at some point, annoying enough to search for a solution.

As it turns out, this message appears when you have Clang libraries installed. QtCreator detects that and automatically uses Clang to parse the source code and provide inline error messages and code completion.

You can get rid of this error when you explicitly add the STL header files’ include-path to your project. In my case, I have added the following to my *.pro file.

unix {
    INCLUDEPATH += /usr/lib/gcc/x86_64-linux-gnu/9/include
}

CMake on Windows 10 with Qt5 Error: CMAKE_PREFIX_PATH

Quick one: Set the proper CMAKE_PREFIX_PATH value for Qt5 development on Windows 10 with MinGW and CMake.

Here is the lovely error you get from CMake.

CMake Error at CMakeLists.txt:13 (find_package):
 By not providing "FindQt5.cmake" in CMAKE_MODULE_PATH this project 
has
 asked CMake to find a package configuration file provided by "Qt5", 
but
 CMake did not find one.

 Could not find a package configuration file provided by "Qt5" with 
any of
 the following names:

 Qt5Config.cmake
 qt5-config.cmake

 Add the installation prefix of "Qt5" to CMAKE_PREFIX_PATH or set 
"Qt5_DIR"
 to a directory containing one of the above files. If "Qt5" provides a
 separate development package or SDK, be sure it has been installed.

CMake asks you to tell it where to find the Qt5 *.cmake configuration files. One option is to set the CMAKE_PREFIX_PATH variable. The Qt5 documentation has the following to say about that:

The easiest way to use CMake is to set the CMAKE_PREFIX_PATH environment variable to the install prefix of Qt 5.

Now, what the hell is the "install prefix"? It turns out it is the path where you can find the compiler-specific Qt binaries, include files and the other stuff. In my case, using MinGW, it is located at "C:\Apps\Qt\5.14.2\mingw73_64". If you are using the MSVC binaries, select the corresponding directory, probably something like "C:\Apps\Qt\5.14.2\msvc_2017".