I am investigating the last remaining modules-related issue on OSX. It pointed a very interesting bug. The output is from ROOT build with -Druntime_cxxmodules=Off.
If I include the header MacTypes.h which defines the struct Point, and normalize the name for ::Point_t, then it suddenly starts resolving things wrongly. I believe something wrong happens here: https://github.com/root-project/root/blob/master/core/foundation/src/TClassEdit.cxx#L1489-L1492
This example a simplified version from https://github.com/root-project/roottest/blob/master/root/meta/naming/execResolveTypedef.cxx#L162 (causing the test failure from the experimental modules nightly build http://cdash.cern.ch/viewTest.php?onlyfailed&buildid=815546)
This issue seems quite severe for ROOT. Also, it is the last issue which prevents us to enable modules on OSX by default.