PatternSynonyms/OverloadedStrings regression in GHC HEAD

purescript-0.10.7 fails to build with GHC HEAD at the moment due to an apparent regression in the way pattern synonyms and overloaded strings interact. Here is a simplified example:

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE PatternSynonyms #-}
module Bug where

import Data.ByteString

newtype ProperName =
  ProperName { runProperName :: ByteString
               -- purescript actually uses the Text type, but this works
               -- just as well for the purposes of illustrating the bug
             }
newtype ModuleName = ModuleName [ProperName]

pattern TypeDataSymbol :: ModuleName
pattern TypeDataSymbol = ModuleName [ProperName "Type",ProperName "Data", ProperName "Symbol"]

Compiling this with GHC 7.10.3 or 8.0.2 works without issue. In GHC HEAD, if you compile this with optimization enabled, it'll trigger a GHC panic:

$ ~/Software/ghc5/inplace/bin/ghc-stage2 -O1 -fforce-recomp Bug.hs                                                                     
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )                                     
ghc-stage2: panic! (the 'impossible' happened)                                            
  (GHC version 8.1.20170303 for x86_64-unknown-linux):                                    
        isUnliftedType                                                                    
  r_a28T :: TYPE rep_a28S                                                                 
  Call stack:                                                                             
      CallStack (from HasCallStack):                                                      
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1191:58 in ghc:Outputable                                                                                    
        callStackDoc, called at compiler/utils/Outputable.hs:1195:37 in ghc:Outputable    
        pprPanic, called at compiler/types/Type.hs:1961:10 in ghc:Type

The presence of optimization is crucial for reproducing this bug, as compiling it with -O0 does not trigger the panic.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler
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