[hist] v5 legacy headers: move to ROOT subfolder

[ferdymercury]

a cryptic "v5" folder is another of the issues for Debian when make-installing ROOT.

Move those legacy headers to ROOT subfolder and provide fallback until fully unsupporting the short-path.

[github-actions]

Test Results

    24 files      24 suites   3d 23h 41m 35s ⏱️
 3 870 tests  3 867 ✅ 0 💤 3 ❌
83 755 runs  83 751 ✅ 0 💤 4 ❌

For more details on these failures, see this check.

Results for commit 3d1a8b9.

♻️ This comment has been updated with latest results.

[guitargeek]

So far, all headers in the ROOT/ subfolder have the .hxx suffix. What would speak against doing this here too, since we're already moving the header?

[guitargeek]

@ferdymercury LGTM in principle, but the CI is red...

[ferdymercury]

but the CI is red...

Sorry, fixed now...

[pcanal]

So far, all headers in the ROOT/ subfolder have the .hxx suffix. What would speak against doing this here too, since we're already moving the header?

On the other hand, those header files do not define 'modern' interfaces (literally the opposite :)), so we probably have to have a short discussion on what is the right path for all the old headers (including TObject.h etc.).

If/since a .hxx suffix is a requirement for going under ROOT/ and that it sort-of signal modern interfaces (or maybe it does not and the prefix T is enough of a counter-clue), it would seen to not belong in ROOT/.

One advantage of keeping the .h is the user would have the 'other' option of doing -I$ROOTSYS/include/ROOT -I$ROOTSYS/include to remove the warning.

[ferdymercury]

If/since a .hxx suffix is a requirement

In my opinion, hxx should be used for C++ headers and h for C headers, independently on the modernity level. So I would not see a problem of moving .h headers into ROOT.
It would align also with clang's policy, otherwise it emits
warning: treating 'c-header' input as 'c++-header' when in C++ mode, this behavior is deprecated [-Wdeprecated]
when generating depfiles.

That being said, the main purpose of this PR was to move it, not to rename the extension, so I am happy to fix either way once there is consensus with @guitargeek and @pcanal :).

[hageboeck]

If/since a .hxx suffix is a requirement for going under ROOT/ and that it sort-of signal modern interfaces (or maybe it does not and the prefix T is enough of a counter-clue), it would seen to not belong in ROOT/.

I have never seen/heard this being a requirement for being in ROOT/. I can also see your point of using -I<prefix>/ROOT, and the header would be found.
For me, this makes .h the preferable option, especially since these headers are already in v5, so they can have a v5 naming convention.

So far, all headers in the ROOT/ subfolder have the .hxx suffix. What would speak against doing this here too, since we're already moving the header?

Breaking code would speak against it. You could run the same code by adding -I<prefix>/ROOT to your build system.

@guitargeek @pcanal, vote?

[guitargeek]

Actually I'm also for .h. I just didn't want to be the guy who approves a PR that sets a precedent for a new pattern, in this case ROOT/*.hxx. That's why I raised that comment. But I could also get behind .hxx.