[GHC] #13755: GHC-8.0.2+ spits out $dm names in error messages

[GHC]

#13755: GHC-8.0.2+ spits out $dm names in error messages
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        Reporter:  zilinc            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10087            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Fixing this infelicity isn't fundamentally hard, but it's a bit fiddly.  I
 can advise if anyone (Ryan?) wants to take it up.

 The challenge is this:

 * In an instance declaration, the default method
 {{{
 reflextive = $dgmreflective @ty
 }}}
  is generated as `HsSyn Name`; and then fed to the type checker.  There
 are excellent reasons for doing this, rather than (say) generating
 something in already-typechecked form.

 * (Something similar happens for ordinary default methods, but it'd be
 `$dmreflective`.

 * When typechecking this binding the typechecker doesn't a-prori know that
 it's special, with a funny name in it.

 * So the typechecker needs to "know" somehow that this `Id` is special.
 One way to do that would be to add a form of `IdDetails` to say "I'm a
 default method Id".  Then the "arising from ..." stuff could print a
 better message.

 * That would not be hard; these Ids are built in one place, by
 `mkDefaultMethodIds`.

 I have not worked out all the details, but am happy to advise.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13755#comment:5>
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