"Inaccessible code" error reported in wrong place

Here is my file Bug.hs:

{-# LANGUAGE GADTs, TypeFamilies #-}

module Bug where

data X a where
  XInt :: X Int
  XBool :: X Bool
  XChar :: X Char

type family Frob a where
  Frob Int = Int
  Frob x   = Char

frob :: X a -> X (Frob a)
frob XInt = XInt
frob _    = XChar

Compiling this file produces the error

Bug.hs:15:6:
    Couldn't match type ‛Int’ with ‛Char’
    Inaccessible code in
      a pattern with constructor XInt :: X Int, in an equation for ‛frob’
    In the pattern: XInt
    In an equation for ‛frob’: frob XInt = XInt

The line/column number single out the pattern XInt in the first clause of the function frob. But, the real problem (as I see it), is the right-hand side of the second clause of frob. Indeed, when I comment out the second line of the function, the error goes away, even though it was reported on the first line.

I do not believe that this error is caused by closed type families, per se, because I have run across it without them, just in code that was too hard to pare down enough to make a bug report out of.

This was tested on 7.7.20130702.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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