[GHC] #13609: regression: `ANN` pragmas for TH-less GHCs no longer ignored in GHC 8.2.1

[GHC]

#13609: regression: `ANN` pragmas for TH-less GHCs no longer ignored in GHC 8.2.1
-------------------------------------+-------------------------------------
           Reporter:  hvr            |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.2.1-rc1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Consider

 {{{#!hs
 module M where

 {-# ANN myId "HLint: ignore" #-}
 myId :: a -> a
 myId x = x
 }}}

 GHC 8.0.2 would simply ignore them:

 {{{
 $ ghc-8.0.2 -c M.hs

 M.hs:3:1: warning:
     Ignoring ANN annotation, because this is a stage-1 compiler or doesn't
 support GHCi
 }}}

 however, with a recent GHC 8.2 snapshot this just fails with

 {{{
 $ ghc-8.2 -c M.hs

 ghc: this operation requires -fexternal-interpreter
 }}}

 This is a serious problem because `hlint` annotations are quite popular,
 and this would require us to retrofit `other-extensions: TemplateHaskell`
 into  the meta-data of quite a few packages on Hackage to declare that a
 package no longer works with a TH-less GHC (which would basically mean
 that ports of GHC such as the one for AIX that don't support TH will
 become very inconvenient to use and require to pester package authors to
 explicitly guard `ANN` pragmas with CPP... which I'm not looking forward
 to ;-) ).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13609>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler