The reason is the DDoc file called phobos.d in Phobos root. DMD seems to accept the import statement, but crashes during importAll.
I can reproduce it.
https://github.com/D-Programming-Language/dmd/commit/a57cebb1e54a72840a9b14bfba8f87a2b54b9ecd https://github.com/D-Programming-Language/dmd/commit/b341bb7c76beb8880938c7f5815998a6c8aa6f7a