PatternSynonyms regression in GHC HEAD (expectJust mkOneConFull)

This code works in GHC 8.2:

{-# LANGUAGE PatternSynonyms #-}
module Bug where

data Foo a = Foo1 a | Foo2 a

pattern MyFoo2 :: Int -> Foo Int
pattern MyFoo2 i = Foo2 i

{-# COMPLETE Foo1, MyFoo2 #-}

f :: Foo a -> a
f (Foo1 x) = x

But it throws a compile-time exception on GHC HEAD:

$ ghc3/inplace/bin/ghc-stage2 Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.3.20170815 for x86_64-unknown-linux):
        expectJust mkOneConFull
CallStack (from HasCallStack):
  error, called at compiler/utils/Maybes.hs:53:27 in ghc:Maybes
  expectJust, called at compiler/deSugar/Check.hs:1128:37 in ghc:Check

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler
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