Ryan Scott authored Oct 10, 2019 and Marge Bot committed Oct 12, 2019

Issue #17339 was caused by using a slightly different version of
`newDFunName` for derived instances that, confusingly enough, did not
take all arguments to the class into account when generating the
`DFun` name. I cannot think of any good reason for doing this, so
this patch uses `newDFunName` uniformly for both derived instances
and manually written instances alike.

Fixes #17339.