[GHC] #14365: Panic with (bogus?) deriving in hs-boot: newTyConEtadArity

[GHC]

#14365: Panic with (bogus?) deriving in hs-boot: newTyConEtadArity
-------------------------------------+-------------------------------------
        Reporter:  happykitten       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4102
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"101a8c770b9d3abd57ff289bffea3d838cf25c80/ghc" 101a8c7/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="101a8c770b9d3abd57ff289bffea3d838cf25c80"
 Error when deriving instances in hs-boot files

 Summary:
 According to the GHC users' guide, one cannot derive
 instances for data types in `.hs-boot` files.
 However, GHC was not enforcing this in practice, which led to
 #14365.

 Fix this by actually throwing an error if a derived instance is
 detected in an `.hs-boot` file (and recommend how to fix it in the
 error message.)

 Test Plan: make test TEST=T14365

 Reviewers: ezyang, austin, bgamari, simonpj

 Reviewed By: simonpj

 Subscribers: simonpj, rwbarton, thomie

 GHC Trac Issues: #14365

 Differential Revision: https://phabricator.haskell.org/D4102
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14365#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler