This reverts commit 21f0b3a6220fdafb8f6aad6855de89d8cdde0e1b.
Rationale:
The absence of clause guard was here done *on purpose*. Indeed,
introducing a clause guard here forces either duplication or the use
of a wildcard which is not "future proof".
Should we make a change to that one day (e.g. add a new variant to
TraceLevel), we won't get any compiler warning and we'll very likely
forget to update that particular section of the code.
So as much as possible, enforce complete pattern-match on variants
make for code that is easier to maintain in the long-run.
2b4137dc24