Ryan Scott authored Feb 05, 2019 and Marge Bot committed Feb 06, 2019

Trac #16287 shows that we were checking for unsaturated type synonym
arguments (in `:kind`) when the argument was to a type synonym, but
_not_ when the argument was to some other form of type constructor,
such as a data type. The solution is to use the machinery that
rejects unsaturated type synonym arguments (previously confined to
`check_syn_tc_app`) to `check_arg_type`, which checks these other
forms of arguments. While I was in town, I cleaned up
`check_syn_tc_app` a bit to only invoke `check_arg_type` so as to
minimize the number of different code paths that that function could
go down.