In GL/glcorearb.h, the following comment can be found.
Do not #include both GL/glcorearb.h and either of GL/gl.h or GL/glext.h in the same source file.
However, this is the only way I can get my code to work. I have in display.h a declaration of wglCreateContextAttribsARB. This requires GL/wglext.h. However, GL/wglext.h gets undefined errors unless GL/glcorearb.h or GL/gl.h is included.
What is the problem here? Does 'source file' perhaps only refer to the .cpp (which worked fine anyway)?