Wildcard patterns and COMPLETE sets can lead to misleading redundant pattern-match warnings

Consider this program:

{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns #-}
{-# OPTIONS_GHC -Wincomplete-patterns #-}
module Bug where

data Boolean = F | T
  deriving Eq

pattern TooGoodToBeTrue :: Boolean
pattern TooGoodToBeTrue <- ((== T) -> True)
  where
    TooGoodToBeTrue = T
{-# COMPLETE F, TooGoodToBeTrue #-}

catchAll :: Boolean -> Int
catchAll F               = 0
catchAll TooGoodToBeTrue = 1

This compiles with no warnings with -Wall. But if you tweak catchAll to add a catch-all case at the end:

{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns #-}
{-# OPTIONS_GHC -Wincomplete-patterns #-}
module Bug where

data Boolean = F | T
  deriving Eq

pattern TooGoodToBeTrue :: Boolean
pattern TooGoodToBeTrue <- ((== T) -> True)
  where
    TooGoodToBeTrue = T
{-# COMPLETE F, TooGoodToBeTrue #-}

catchAll :: Boolean -> Int
catchAll F               = 0
catchAll TooGoodToBeTrue = 1
catchAll _               = error "impossible"

Then if you compile it with -Wall, you'll get a very misleading warning:

$ ~/Software/ghc2/inplace/bin/ghc-stage2 --interactive Bug.hs -Wall
GHCi, version 8.1.20170228: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:17:1: warning: [-Woverlapping-patterns]
    Pattern match is redundant
    In an equation for ‘catchAll’: catchAll TooGoodToBeTrue = ...
   |
17 | catchAll TooGoodToBeTrue = 1
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I would have expected the warning to be on the catchAll _ = error "impossible" case!

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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CC	mpickering
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