Erroneous warnings for -XPatternGuards

As nonstandard PatternGuards I get a meaningless warning:

walltest.hs:8:10:
    Warning: Pattern match(es) are non-exhaustive
             In the definition of `n': Patterns not matched:
Ok, modules loaded: Main.

{-# LANGUAGE PatternGuards #-}
{-# OPTIONS -Wall #-}

data T = A Int | B Int

funny :: T -> Int
funny t = n
    where n | A x <- t = x
            | B x <- t = x

But none of that happens when using a case expression for the same thing:

{-# OPTIONS -Wall #-}

data T = A Int | B Int

funny :: T -> Int
funny t = n
    where n = case t of
                (A x) -> x
                (B x) -> x

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture