Wrong diagnostics for instance signature mismatch

Consider:

---

{-# LANGUAGE InstanceSigs, DataKinds, KindSignatures, GADTs #-}

data DayKind = Work | Rest

data Day :: DayKind -> * where

Mon :: Day Work Sat :: Day Rest

instance Num (Day c) where

fromInteger :: Int -> Day p fromInteger 1 = Mon fromInteger 6 = Sat

---

I get

$ ~/bin/ghci NumGADT.hs GHCi, version 7.5.20120410: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( NumGADT.hs, interpreted )

NumGADT.hs:10:18:

Method signature does not match class; it should be

fromInteger	forall (p :: DayKind). Int -\> Day p
	In the instance declaration for \`Num (Day c)'

Failed, modules loaded: none. Prelude>

It appears to me that instead of telling me the expected signature GHCi simply echoes what I have provided. This is not really useful ;-)

Trac metadata

Trac field	Value
Version	7.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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