It is fixing:
..root/tree/dataframe/src/RDataFrame.cxx:789:6: error: reference to 'RDF' is ambiguous
   : RDF::RInterface<RDFDetail::RLoopManager>(
     ^
In module 'ROOTDataFrame' imported ..root/tree/dataframe/src/RDataFrame.cxx:14:
..include/ROOT/RCutFlowReport.hxx:29:11: note: candidate found by name lookup is 'ROOT::RDF'
namespace RDF {
          ^
..root/tree/dataframe/src/RDataFrame.cxx:721:11: note: candidate found by name lookup is 'ROOT::Detail::RDF'
namespace RDF {
          ^
..tree/dataframe/src/RDataFrame.cxx:789:11: error: member initializer (null) does not name a non-static data member or base class
   : RDF::RInterface<RDFDetail::RLoopManager>(
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This reverts commit 80b1a73b.

The patch triggers a pre-existing bug on fedora28 and gcc8.1

This is a better implementation over doing #ifdef R__USE__CXXMODULES
everywhere.

Such pictures can (now) be displayed with `\macro_image (picture_name.png[.pdf])`

where possible without too many hacks.

reorder images and code, add output, make sure canvases can be
properly saved. Also trim out useless code to minimise the examples.

if the size is bigger than the default value. Document better the behaviour, too.

This error: http://cdash.cern.ch/viewBuildError.php?buildid=567476
was caused because modules couldn't merge definition of struct __pthread_internal_list

This is to enable the supression of file modification error and module
relocation error provided by Clang. File modification error is emitted
when source file is changed after implicit pcm was generated, but what
we want for Clang is to just re-generate new pcm and replace it, rather than
emitting errors.

but rather for the missing dictionary of T given the way in which we
treat unique_ptrs.

Fixes builds with command line tools and macos 10.14.

Previously, [system] was used to mark modules which were supporsed be
implicitly built by rootcling for runtime modules. This was a workaround, and it was causing
SourceLocation error in cxxmodules as it is also using the same
modulemap.

To remove the workaround, we are introducing a new convention that modules
start with ROOT_ is treated as "ROOT system modules", which can be build
implicitly.

This solves the 2nd schema evolution problem seen at https://github.com/cms-sw/cmssw/pull/22594#issuecomment-422105335.

This was leading to a crash seem a rule was executed for a non-existent data member.

When looking for the 'onfile' StreamerInfo we actually mean to find the StreamerInfo that will help load from
the file representation to the in memory represenation, so if the target class is not the same as the original
class we actually need a ConversionStreamerInfo.

This is needed as ResetInitInfo is usually called after we changed the target class and thus the current class.
Set it to nullptr to let the normal operation recalculated the value.

To properly 'reset' and 'restart' the StreamerInfo processing in TBranchElement, we need
to call ResetInitInfo in more places

since ROOT stopped compiling.

Revert "Remove dead code I know I can remove"

This reverts commit c04d4cf6.

Revert "[Math/Fit] Update documentation"

This reverts commit 76f58d3a.

Revert "Merge old PoissonLogL functions into new interfaces"

This reverts commit b1cd6e68.

Revert "Merge old LogL functions into new interfaces"

This reverts commit 059092a7.

Revert "Merge old functions into new interfaces"

This reverts commit ccecadaf.

Revert "Move template specification definitions to source file"

This reverts commit 5192cf5b.

Revert "Fix undefined default initialization"

This reverts commit 92af804e.

Revert "Fix bug in gradient evaluation"

This reverts commit cbc475ca.

Revert "Apply clang-format"

This reverts commit 7007245d.

Revert "Improve loop performance"

This reverts commit 9ddf280f.

Revert "Fix iterator type"

This reverts commit 271738b8.

Revert "Fix non initialization warning"

This reverts commit 5b576e20.

Revert "Discard unnecessary operations"

This reverts commit 2dd87289.

Revert "Add braces for consistency"

This reverts commit dcc37acf.

Revert "Update infinite check functions"

This reverts commit 06c127da.

Revert "Remove unnecessary specialization"

This reverts commit 7e767e98.

Revert "Remove unnecessary cast"

This reverts commit c8020a0f.

Revert "Fix comment"

This reverts commit 711185dc.

Revert "Add missing include"

This reverts commit d6bfc08a.

Revert "Factor out objective functions from FitUtil"

This reverts commit c498e8c8.

Revert "Fix header comment"

This reverts commit d30691e4.

Added dependencies to onepcm target to protect from running to early in build system.
ROOT_LIBRARY_TARGETS records all targets that is used for generation of allDict.cxx.pch.

and other minor improvements.

With 16 threads, the cache is simply too small.
These are thread specific values anyway.