[GHC] #13591: "*** Exception: expectJust showModule" in ghci with hs-boot

[GHC]

#13591: "*** Exception: expectJust showModule" in ghci with hs-boot
-------------------------------------+-------------------------------------
           Reporter:  lspitzner      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  GHCi           |           Version:  8.2.1-rc1
           Keywords:  ghci hs-boot   |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 ghci runs into an exception if there is an error in Foo.hs when there
 exists a
 Foo.hs-boot (which compiles fine).

 - First.hs-boot
 {{{
 module First where
 one :: Int
 }}}

 - Second.hs
 {{{
 module Second where
 import {-# SOURCE #-} First
 two :: Int
 two = one + 1
 }}}

 - First.hs
 {{{
 module First where
 import Second
 one :: Int
 one = _
 }}}

 {{{
 > ghci
 Prelude> :l First
 [1 of 3] Compiling First[boot]      ( First.hs-boot, interpreted )
 [2 of 3] Compiling Second           ( Second.hs, interpreted )
 [3 of 3] Compiling First            ( First.hs, interpreted )

 First.hs:6:7: error:
     • Found hole: _ :: Int
     • In the expression: _
       In an equation for ‘one’: one = _
     • Relevant bindings include one :: Int (bound at First.hs:6:1)
 *** Exception: expectJust showModule
 CallStack (from HasCallStack):
   error, called at compiler/utils/Maybes.hs:48:27 in ghc:Maybes
 }}}

 This happens on ghc-8.0.1, 8.0.2 and 8.2.1-rc1. The above output is from
 8.0.2, but the error seems to be essentially the same on 8.2.1-rc1. It did
 not happen on 7.10.3.

 While ghci continues to work, the exception appears to clear the loaded-
 modules list, which in turn breaks tooling (ghcid here, which hangs as a
 consequence). The expected behaviour would be that other modules (which
 contain no errors) remain loaded.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13591>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler