Bad pattern match code generated for list comprehension

[erikd]

This code:

main ()
 = println $ show [ x |  xs <- [ [(1,2),(3,4)], [(5,4),(3,2)] ], (3,x) <- xs ]

compiles, but when run terminates with the error:

*** DDC RTS: Non-exhaustive patterns in ?:0:0

This is actually two bugs:

• Failure to give a valid source position (?:0:0) for the error.
• Non-exhaustive pattern match.

[erikd]

The pattern match inside the list comprehension gets desugared to (from -dump-desugar output):

Data.List.concatMap 
        (\xSD0 ->
        match {
                | Data.Tuple.Tuple2 {.0 = xSD2, .1 = Main.x} <- xSD0
                , Base.True {} <- Class.Eq.(==) (3i32) xSD2
                = Data.List.Cons Main.x Data.List.Nil;
        }) Main.xs;

This would be correct for a standard pattern match, not for inside a list comprehension. Inside an LC, individual failed matches should result in an empty list element.

[batterseapower]

I have a patch for this at http://www.omega-prime.co.uk/files/ddc/175.dpatch which you may wish to apply

I would attach it to the ticket but Trac said "permission denied" when I tried.