Generalisation behaviour of partial type signatures

{-# LANGUAGE InstanceSigs, PartialTypeSignatures #-}

import Foreign.C.Types
import Foreign.Storable
import Foreign.Ptr

data CTimeval = MkCTimeval CLong CLong

peek :: Ptr CTimeval -> IO CTimeval
peek ptr = do
  s   <- peekElemOff (castPtr ptr) 0 :: _ _ 
  mus <- peekElemOff (castPtr ptr) 1
  return (MkCTimeval s mus)

Fails with

baldur@Loki:~$ ghci -ignore-dot-ghci /tmp/tLcV.hs 
GHCi, version 8.1.20160117: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( /tmp/tLcV.hs, interpreted )

/tmp/tLcV.hs:11:10: error:
    • No instance for (Storable t)
      Possible fix:
        add (Storable t) to the context of
          an expression type signature:
            IO t
    • In a stmt of a 'do' block:
        s <- peekElemOff (castPtr ptr) 0 :: _ _
      In the expression:
        do { s <- peekElemOff (castPtr ptr) 0 :: _ _;
             mus <- peekElemOff (castPtr ptr) 1;
             return (MkCTimeval s mus) }
      In an equation for ‘peek’:
          peek ptr
            = do { s <- peekElemOff (castPtr ptr) 0 :: _ _;
                   mus <- peekElemOff (castPtr ptr) 1;
                   return (MkCTimeval s mus) }
Failed, modules loaded: none.

while

{-# LANGUAGE InstanceSigs, PartialTypeSignatures #-}

import Foreign.C.Types
import Foreign.Storable
import Foreign.Ptr

data CTimeval = MkCTimeval CLong CLong

peek :: Ptr CTimeval -> IO CTimeval
peek ptr = do
  s   <- peekElemOff (castPtr ptr) 0 :: _ CLong 
  mus <- peekElemOff (castPtr ptr) 1
  return (MkCTimeval s mus)

succeeds. Is this expected?

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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