[GHC] #15523: GHC Panic on malformed newtype and StrictData

[GHC]

#15523: GHC Panic on malformed newtype and StrictData
-------------------------------------+-------------------------------------
           Reporter:  rdnetto        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following source causes a panic:
 {{{#!hs
 {-# LANGUAGE StrictData #-}

 module Main where

 newtype Duration = Foo
 data Literal = LitDuration Duration
 }}}

 Result:
 {{{
 [1 of 1] Compiling Main             ( src/Main.hs, .stack-work/dist/x86_64
 -linux-tinfo6/Cabal-2.2.0.1/build/tcalc/tcalc-tmp/Main.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.4.3 for x86_64-unknown-linux):
         mkNewTyConRhs
   Foo []
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in
 ghc:Outputable
         pprPanic, called at compiler/iface/BuildTyCl.hs:77:27 in
 ghc:BuildTyCl

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 If we remove the StrictData pragma, then we get a syntax error as
 expected:
 {{{
 /home/reuben/tcalc/src/Main.hs:3:20: error:
     • The constructor of a newtype must have exactly one field
         but ‘Foo’ has none
     • In the definition of data constructor ‘Foo’
       In the newtype declaration for ‘Duration’
   |
 3 | newtype Duration = Foo
   |                    ^^^
 }}}

 I was able to reproduce this with GHC 8.4.3 and 8.2.2 (using Stack LTS
 snapshots). GHC 8.0.2 is not affected.

 A test case buildable with Stack can be found at
 https://github.com/rdnetto/tcalc/tree/ghc-bug (i.e. on the ghc-bug
 branch).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15523>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler