Better error message regarding type synonyms and PolyKinds

Followup of https://mail.haskell.org/pipermail/haskell-cafe/2015-July/120505.html

ghci> :set -XPolyKinds
ghci> type Test a = a
ghci> type Huh = Test

<interactive>:6:1:
    Type synonym ‘Test’ should have 2 arguments, but has been given 1
    In the type declaration for ‘Huh’

(Notice the number of arguments reported)

Jonas Scholl at haskell-cafe suggested that the additional invisible argument is a kind argument, which indeed looks to be the case, as he demonstrated:

Prelude> :set -XPolyKinds -XDataKinds -XKindSignatures
Prelude> type Test a = a
Prelude> type Huh a = Test (a :: Bool)
Prelude> type Bar = Huh

<interactive>:5:1:
    Type synonym ‘Huh’ should have 1 argument, but has been given none
    In the type declaration for ‘Bar’

A better error message may be desirable here.

Trac metadata

Trac field	Value
Version	7.10.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
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