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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.
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