Finally found it. Couldn't find it because the French naming doesn't make it obvious.
The problem is caused by a linking problem with unit test code coverage. It looks like it is a problem inside vibe.d. I wished coverage was working because I like this feature.
Construction : testvibe (Default)
Performing main compilation...
dub build "testvibe" "--build=cov"
Building package testvibe in /home/XXX/Sources/testVibe/
Performing "cov" build using dmd for x8664.
vibe-d:utils 0.7.28: target for configuration "library" is up to date.
vibe-d:data 0.7.28: target for configuration "library" is up to date.
vibe-d:core 0.7.28: target for configuration "libevent" is up to date.
vibe-d:http 0.7.28: target for configuration "library" is up to date.
vibe-d:diet 0.7.28: target for configuration "library" is up to date.
vibe-d:mail 0.7.28: target for configuration "library" is up to date.
vibe-d:mongodb 0.7.28: target for configuration "library" is up to date.
vibe-d:redis 0.7.28: target for configuration "library" is up to date.
vibe-d:web 0.7.28: target for configuration "library" is up to date.
vibe-d 0.7.28: target for configuration "libevent" is up to date.
testvibe ~master: building configuration "application"...
Linking...
../../.dub/packages/vibe-d-0.7.28/libvibe-dcore.a(task.o): dans la fonction « D4vibe4core4sync125T13performLockedS62D4vibe4core4task12MessageQueue5clearMFZ9lambda1MFNaNbNiNfZvTC4vibe4core4sync22InterruptibleTaskMutexZ13performLockedMFNfC4vibe4core4sync22InterruptibleTaskMutexZv »:
/home/XXX/Sources/testVibe/../../.dub/packages/vibe-d-0.7.28/source/vibe/core/sync.d:97: référence indéfinie vers « coverage »
/home/XXX/Sources/testVibe/../../.dub/packages/vibe-d-0.7.28/source/vibe/core/sync.d:99: référence indéfinie vers « coverage »
/home/XXX/Sources/testVibe/../../.dub/packages/vibe-d-0.7.28/source/vibe/core/sync.d:101: référence indéfinie vers « coverage »
/home/XXX/Sources/testVibe/../../.dub/packages/vibe-d-0.7.28/source/vibe/core/sync.d:101: référence indéfinie vers « coverage »
../../.dub/packages/vibe-d-0.7.28/libvibe-d_core.a(task.o): dans la fonction « _D4vibe4core4sync158T13performLockedS95D4vibe4core4task12MessageQueue4sendMFS3std7variant18T8VariantNVmi32Z8VariantNZ--- errorlevel 1
9lambda2MFZvTC4vibe4core4sync22InterruptibleTaskMutexZ13performLockedMFC4vibe4core4sync22InterruptibleTaskMutexZv »:
/home/XXX/Sources/testVibe/../../.dub/packages/vibe-d-0.7.28/source/vibe/core/sync.d:97: référence indéfinie vers « coverage »
../../.dub/packages/vibe-d-0.7.28/libvibe-dcore.a(task.o):/home/XXX/Sources/testVibe/../../.dub/packages/vibe-d-0.7.28/source/vibe/core/sync.d:99: encore plus de références indéfinies suivent vers « __coverage »
collect2: error: ld returned 1 exit status
dmd failed with exit code 1.
Exit code 2
Compilation terminée -- 1 erreur, 0 avertissement
---------------------- Fini ----------------------
Construire : 1 erreur, 0 avertissement