[Build system] ROOT installs its builtins directly into `CMAKE_INSTALL_PREFIX`.

[hageboeck]

• Checked for duplicates

Describe the bug

When you install a ROOT with builtins, it also installs the builtin headers (+ libraries probably) into the system directory.
This might be at least surprising, if not annoying.

Expected behavior

If builtins are needed, they could go into ${CMAKE_INSTALL_PREFIX}/include/ROOT_builtins} or similar.

This unfortunately requires going through all builtins, and changing their install locations, but I don't see a way around that.

To Reproduce

cmake -DCMAKE_INSTALL_PREFIX=/tmp/dummyInstallation <path/to/root>
make -j install

Setup

Master, linux.

[eguiraud]

The headers of a builtin nlohmann/json are installed at ${CMAKE_INSTALL_PREFIX}/include/nlohmann, which (as far as installing third-party stuff goes) is probably ok?

Or you would rather have ROOT install those headers at ${CMAKE_INSTALL_PREFIX}/include/ROOT_builtins/nlohmann? If this was the case, projects building against ROOT would not find the builtins anymore, at least not without extra care.

[hageboeck]

I guess the question is whether ROOT should interfere with what you installed in your system. Let's say you have something installed in /usr/include/nlohmann. Now you install ROOT, and for some reason it came with a builtin nlohmann_json (e.g. because of a different version). Now, everything in your system that was relying on the original nlohman_json version is broken.
I thought that as far as reasonably possible, ROOT should not interfere with what you install in your system (i.e. it should leave all headers and libraries untouched), unless they are ROOT's own headers/libraries?

You are correct that projects building against that installed ROOT would need to know about the include location, but I would think that this is ROOT's job (by exporting CMake targets such as ROOT::nlohmann).

Maybe that's what users expect of ROOT, but I was at least surprised to have stuff installed that I didn't ask for.

Let me rephrase this, maybe:
You install X from source. Would you expect that this also installs Y, and possibly overwrites / corrupts any existing Ys?