I can reproduce on both macOS 10.13 and 10.14. I too have CC/CXX set to the clang recommended and installed by |mach bootstrap|. Using the system clang works around this problem, which is presumably why it doesn't happen in CI.
Bug 1536486 Comment 1 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
I can reproduce on both macOS 10.13 and 10.14. I too have CC/CXX set to the clang recommended and installed by `mach bootstrap`. Using the system clang works around this problem, which is presumably why it doesn't happen in CI.