File extension .ic which is used for some include files is not a commonly recognized as C/C++ source file extension, thus IDE such as Visual Studio, by default does not use syntax highlighting, and does not index them (so search for symbols is broken)
moreover, cmake when creating VS project, does not correctly place those .ic files into either Source Files, or Header Files.

Solution is to use a more conventional extension. While some people (including myself) would be fine with .h, commonly used for include files, there also exists a different opinion ,that those includes are somehow different from others, and thus need different extension.

Using .inl (recognized by VS as C/C++ file) is therefore chosen as compromise.