Hi, mongodb 4.2.1 builds fine on macOS 10.13 when compiled with --libc++ using MacPorts clang 7, and also with clang 8 if I apply the fix from SERVER-44270. But using clang 9, the build fails with hundreds of thousands of duplicate symbol errors like this:

duplicate symbol __ZN5mongo13error_details11isNamedCodeILi9EEE in:
build/MP/mongo/shell/dbshell.o
build/MP/mongo/shell/mongodbcr.o
duplicate symbol __ZN5mongo13error_details11isNamedCodeILi199EEE in:
build/MP/mongo/shell/dbshell.o
build/MP/mongo/shell/mongodbcr.o
duplicate symbol __ZN5mongo13error_details11isNamedCodeILi89EEE in:
build/MP/mongo/shell/dbshell.o
build/MP/mongo/shell/mongodbcr.o

My compressed full build log is attached.

Another user reported the same problem when using clang 9 to build mongodb 4.0.13 on OS X 10.11. Here is their compressed full build log if you'd like it.