Yes, Great that is exactly what I meant - installing headers without use of install(FILES or DIRECTORIES, thank you. So now you could see why I am reporting cmake issue. While all works fine per your example, when SHARED is replaced by INTERFACE the headers are not being installed. Such limitation for INTERFACE libraries was not documented . So despite calling install(TARGETS we had to use install(FILES or DIRECTORIES. So first could you please confirm that cmake shall supports installing INTERFACE libraries or point to any limitation int the book or documentation?
Secondly, why the limitation of flat include directory exists? cmake is able to configure build with INTERFACE library with non-flat include structure and installation with EXPORT merely exports build tree to install path. cmake could have installed to directory specified by PUBLIC_HEADERS with all its subdirectories as a default. So why cmake is having problem with this install ?