Somehow I'd forgotten to instantiate the coercion that is stored in a
'NewtypeDerived' constructor in an InstInfo.  The necessary code is
in TcInstDcls.tc_inst_decl2.

The result was ghc: panic! (the 'impossible' happened)
   (GHC version 6.10.3 for x86_64-unknown-linux):
   No match in record selector Var.tcTyVarDetails
because we were looking at an (uninstantiated) TyVar instead of
an (instantiated) TcTyVar.