Simon Peyton Jones authored Jun 03, 2019 and Marge Bot committed Jun 07, 2019

When the canonicaliser rewrites evidence of a Wanted, it
should preserve the ShadowInfo (ctev_nosh) field.  That is,
a WDerive should rewrite to WDerive, and WOnly to WOnly.

Previously we were unconditionally making a WDeriv, thereby
rewriting WOnly to WDeriv.  This bit Nick Frisby (issue #16735)
in the context of his plugin, but we don't have a compact test
case.

The fix is simple, but does involve a bit more plumbing,
to pass the old ShadowInfo around, to use when building
the new Wanted.