[GHC] #15617: Unboxed tuples/sum error message on `a = show 5` in expression evaluation and interactive modes

[GHC]

#15617: Unboxed tuples/sum error message on `a = show 5` in expression evaluation
and interactive modes
-------------------------------------+-------------------------------------
        Reporter:  ChaiTRex          |                Owner:
                                     |  JulianLeviston
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1-beta1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by JulianLeviston):

 I'm leaving this here for myself as WIP notes for later.

 Execution path:

 1. ghc/Main.hs main function
 1. (mode, argv3, flagWarnings) <- parseModeFlags argv2
 1. case mode of ... Right postStartupMode ->
 1. case postStartupMode of ->
 1. Right postLoadMode -> main' postLoadMode dflags argv3 flagWarnings
 1. this hits the main' function after getting a postLoadMode of
 DoInteractive or DoEval
 1. this is the meat, where we're pulling apart the flags then throwing
 them at ghciUI which calls interactiveUI defaultGhciSettings for both
 cases
 1. This function comes from GHCi.UI

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15617#comment:16>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler