John Ericson authored Nov 06, 2019 and Marge Bot committed Jan 16, 2020

Before, it was a panic because it was handled above. But there must have
been an error in my reasoning (another caller?) because #17442 reported
the panic was hit.

But, rather than figuring out what happened, I can just make it
impossible by construction. By adding just a bit more bureaucracy in the
return types, I can handle TagToEnum in the same case as all the others,
so the big case is is now total, and the panic is removed.

Fixes #17442