Allow splitting of TTime objects

[pcanal]

This fixes #12498.

TTime objects were not (anymore) split because TTime is still using the automatically generated but streamer-function based I/O. (The splitting stopped when TTree was upgraded to respect custom streamers).

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu18.04/nortcxxmod, ROOT-ubuntu2004/python3, mac12/noimt, mac11/cxx14, windows10/cxx14
How to customize builds

[github-actions]

Test Results

       2 files         2 suites   15h 34m 9s ⏱️
2 416 tests 2 391 ✔️ 2 💤 23 ❌
4 834 runs  4 790 ✔️ 8 💤 36 ❌

For more details on these failures, see this check.

Results for commit 42d25a9.

[Axel-Naumann]

Is this really forward (and backward) compatible?

Is this class the only case in ROOT?

[pcanal]

Is this really forward (and backward) compatible?

For this class the non-split layout is exactly the same, the only difference is splitting is now allowed. I verified that a non-split file can (of course) be read.

[pcanal]

Is this class the only case in ROOT?

It is 'hard' to say. There about a 1000 pragma link C++ class with a + or -. Those are legitimate if the there is no ClassDef nor Streamer function (in which case there is no alternative to the StreamerInfo based I/O) and for classes which version is 0 (i.e. not intended for storage). I suppose we could write a script to find out.

[pcanal]

So out of 1012 such pragma link, there 199 classes I could not (i.e. I did not build the plugin).
590 classes had a class version of 0 (so it is fine)
150 classes had no ClassDef (so it is fine)
12 were reported as having no Steamer function but inherited from TObject (and upon inspection seemed to have a ClassDef), so it looks a bug in the detection in rootcling for some case of class template instance.

That left 61 potential issue (list below) and from that list only TTime seemed to be both intended for I/O and might be put in a TTree:

Potential issue: TAttImage
Potential issue: TImage
Potential issue: TImagePalette
Potential issue: TPyReturn
Potential issue: TPyArg
Potential issue: TPyReturn
Potential issue: TPyArg
Potential issue: TRecEvent
Potential issue: TRecCmdEvent
Potential issue: TRecExtraEvent
Potential issue: TRecGuiEvent
Potential issue: TRecWinPair
Potential issue: TRecorder
Potential issue: TXMLPlayer
Potential issue: TXMLEngine
Potential issue: TXMLSetup
Potential issue: TXMLFile
Potential issue: TKeyXML
Potential issue: TKeySQL
Potential issue: TSQLClassInfo
Potential issue: TSQLClassColumnInfo
Potential issue: TSQLObjectInfo
Potential issue: TSQLObjectData
Potential issue: TSQLObjectDataPool
Potential issue: TSQLStructure
Potential issue: TSQLColumnData
Potential issue: TSQLTableData
Potential issue: TFree
Potential issue: TFumiliMinimizer
Potential issue: TStructNodeProperty
Potential issue: TGLVertex3
Potential issue: TGLVector3
Potential issue: TGLMatrix
Potential issue: TGLCamera
Potential issue: TGLOrthoCamera
Potential issue: TGLPerspectiveCamera
Potential issue: TGLCameraOverlay
Potential issue: TVirtualMonitoringReader
Potential issue: TUri
Potential issue: TUrl
Potential issue: SysInfo_t
Potential issue: CpuInfo_t
Potential issue: MemInfo_t
Potential issue: ProcInfo_t
Potential issue: TTime
Potential issue: TPluginHandler
Potential issue: TPluginManager
Potential issue: TArray!
Potential issue: TDataType
Potential issue: TGlobal
Potential issue: THostAuth
Potential issue: TProofDesc
Potential issue: TProofNodeInfo
Potential issue: TProofProgressStatus
Potential issue: TProofProgressInfo
Potential issue: TNetFile
Potential issue: TWebFile
Potential issue: TFTP
Potential issue: TSelectorDraw
Potential issue: TSelectorEntries
Potential issue: TChainElement