give better error messages for instance declarations with the wrong number of parameters

Consider the following erroneous program which tries to define an instance of MonadReader.

import Control.Monad.Reader

newtype Reader' r a = Reader' (r -> a)

instance MonadReader (Reader' r)

This instance declaration is wrong, because the MonadReader type class has two parameters. But ghc's error message is not very helpful:

/tmp/err.hs:5:21:
    `Reader' r' is not applied to enough type arguments
    Expected kind `*', but `Reader' r' has kind `* -> *'
    In the instance declaration for `MonadReader (Reader' r)'

While if I give too many arguments, e.g., instance MonadReader r r (Reader' r), I always get a good error message, even if the kinds of some of the leading types are wrong:

    Kind error: `MonadReader' is applied to too many type arguments
    In the instance declaration for `MonadReader r r (Reader' r)'

So it'd be great if the error message for giving too few type arguments could be more like that one.

Trac metadata

Trac field	Value
Version	6.11
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture