unreachable GADT pattern clauses show up as warnings with -Wall

In the following code, there is a line commented out. With it commented out, I get the following warning:

patternbug.hs:10:0:
    Warning: Pattern match(es) are non-exhaustive
	     In the definition of `interpret': Patterns not matched: EVar

With it uncommented, I get the following compiler error:

patternbug.hs:11:10:
    Inaccessible case alternative: Can't match types `()' and `(a, vs)'
    In the pattern: EVar
    In the definition of `interpret':
	interpret EVar = error "unreachable"

The exhaustive pattern-matching algorithm should ignore unreachable cases in its analysis.

{-# OPTIONS_GHC -Wall #-}
{-# LANGUAGE GADTs #-}
module PatternBug where

data Expr a vs where
    EPrim   :: String -> a -> Expr a vs
    EVar    :: Expr a (a,vs)

interpret :: Expr a () -> a
interpret (EPrim _ a) = a
-- interpret EVar = error "unreachable"

Trac metadata

Trac field	Value
Version	6.8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ryani.spam@gmail.com
Operating system	Unknown
Architecture	Unknown