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

mentioned in issue #595 (closed)

changed weight to 5

added Tbug Trac import labels

closed

It's a very special case. For example you might have

... where n | A x <- f t = x
            | B x <- t = x

or

... where n | A x <- t, x>0 = x
            | B x <- t = x

etc. In principle a better exhaustiveness checker could catch some of these cases, and that'd be great. Good suggestion, but it's definitely not simple.

I'm going to close this ticket as a kind-of duplicate, and attach it to the main exhaustiveness checking ticket, namely #595 (closed)

Simon

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedDuplicate

mentioned in commit 8a506104

added Pnormal label

added pattern match warnings label