Non-exhaustive pattern, "Patterns not matched" list is empty

Example:

module Main where

maybeOdd :: Int -> Maybe Int
maybeOdd i = if odd i then Just i else Nothing

main :: IO ()
main = do
  let x = maybeOdd 10

  let a | Just i <- x
        , odd i
        = True

        | Nothing <- x
        = False

  print x
  print a

Warning printed by GHC HEAD:

Exhaustive.hs:10:7: warning:
    Pattern match(es) are non-exhaustive
    In an equation for ‘a’: Patterns not matched:
Linking Exhaustive ...

The problem with this message is; if it couldn't come up with an example unmatched pattern, then how can it know that the pattern is non-exhaustive? If it came up with an example, why is that example not printed?

UPDATE: I just realized it's actually worse that I first thought. If I change a in this example:

  let a | Just i <- x
        = True

This message is printed:

[1 of 1] Compiling Main             ( Exhaustive.hs, Exhaustive.o )

Exhaustive.hs:10:7: warning:
    Pattern match(es) are non-exhaustive
    In an equation for ‘a’: Patterns not matched:

Exhaustive.hs:10:16: warning: Defined but not used: ‘i’
Linking Exhaustive ...

NOTE: Tried with GHC 7.10 too. It seems like in the case where the checks are not exhaustive, both 7.10 and HEAD are giving the same warning(with empty list of non-checked patterns). HEAD is better in detecting exhaustive patterns.

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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