Wrong and redudant `incomplete-patterns` warnings

Summary

Recent GHC versions emit a wrong incomplete-patterns warning. Furthermore, for a similar program that deserves to be warned about, those versions are repetitive when emitting the warning. Earlier GHC versions fail to compile programs of this sort.

Steps to reproduce

A.hs

{-# language PatternSynonyms            #-}
{-# options_ghc -fforce-recomp -Wall    #-}
module A ( pattern P, foo ) where

pattern P :: a -> a
pattern P x = x
{-# complete P #-}
-- {-# complete P :: (,) #-} -- Needed for ghc-8.10.2 and prior.

foo :: ()
foo = case () of
    P _ -> ()

B.hs

{-# language PatternSynonyms            #-}
{-# options_ghc -fforce-recomp -Wall    #-}

module B ( bar ) where

import A ( pattern P )

bar :: ()
bar = case ((), "") of
    P ((), "hello") -> ()

C.hs

{-# language LambdaCase, PatternSynonyms    #-}
{-# options_ghc -fforce-recomp -Wall        #-}

module C ( woe ) where

import A ( pattern P )

woe :: () -> ()
woe = \case
    P _ -> ()

Expected behavior

Do not warn

ghc-8.10.2 and ghc-9.1.0.20201115 emit the same incomplete patterns warning when compiling A.hs, the missing pattern being (). The warning might be unwarranted. On another venue, @sgraf812 said this about that:

The […] example with a COMPLETE P :: () on the other hand has a different quality!

Do not repeat the missing pattern

ghc-8.10.2 and ghc-9.1.0.20201115 correctly warn on B.hs about incomplete patterns but the warning carries redundant elements as shown below:

warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In a case alternative:
        Patterns not matched:
            ((), [])
            ((), [])
            ((), [])
            ((), [])
            ...
  |
  | bar = case ((), "") of
  |       ^^^^^^^^^^^^^^^^...

Sebastian said:

[There are] duplicates the origin of which are unclear to me. I have seen that when view patterns were used, but not with just pattern synonyms. But nothing too serious.

I think it’s related to the fact that we record information about the scrutinee (e.g. that it’s ()) and then don’t store the fact that we matched on P to conclude that the match is complete. If you match on a parameter x :: () instead, you don't get any warning.

I added C.hs to illustrate that last sentence.

Check if there is a regression test

While ghc-8.10.2 wrongly warns on C.hs, saying that the () pattern is not matched, ghc-9.1.0.20201115 does not. C.hs would then serve as a regression test, were there nothing else similar.

Explain why earlier versions can not compile these programs

ghc-8.6.5 and ghc-8.8.4 panic on any of A.hs, B.hs and C.hs.

Environment

• GHC versions used: [8.6.5, 8.8.4, 8.10.2, head (9.1.0.20201115)]

Optional:

• Operating System: GNU/Linux
• System Architecture: x86_64

Cc @int-e @exipipiplus1 @expipiplus1