EmptyCase produces incorrect SrcSpans

If you compile the following program, you'll get a warning for f with an improperly placed SrcSpan:

{-# LANGUAGE EmptyCase #-}
{-# LANGUAGE TypeOperators #-}
{-# OPTIONS_GHC -Wincomplete-patterns #-}
module Bug where

import Data.Type.Equality

can'tHappen :: Int :~: Bool
can'tHappen = undefined

f, g :: Bool -> Bool
f True = case can'tHappen of {}
g True = case () of () -> True

$ /opt/ghc/8.4.2/bin/ghci Bug.hs
GHCi, version 8.4.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:12:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘f’: Patterns not matched: False
   |
12 | f True = case can'tHappen of {}
   | ^^^^^^^^^^^^^

Bug.hs:13:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘g’: Patterns not matched: False
   |
13 | g True = case () of () -> True
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Ok, one module loaded.

Compare that to the warning for g, whose SrcSpan spans the entire clause, as expected.

The bug lies in the parser—patch incoming.

Trac metadata

Trac field	Value
Version	8.4.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Parser)
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