[GHC] #13266: Source locations from signature merging/matching are bad

[GHC]

#13266: Source locations from signature merging/matching are bad
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           Reporter:  ezyang         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:  backpack       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Example output:

 {{{
 /tmp/ghc3555_0/ghc_7.hscpp:1:1: error:
     No instance for (GHC.Enum.Enum t)
       arising when attempting to show that
       instance [safe] MonadHold t (PushM t)
         -- Defined in ‘z-reflex-z-common-0.5.0:Reflex.NewPure’
       is provided by ‘z-reflex-z-common-0.5.0:Reflex.NewPure’
     Possible fix:
       add (GHC.Enum.Enum t) to the context of the instance declaration
   |
 1 | # 1 "indef/Reflex/Sig.hsig"
 }}}

 Unfortunately, this is not so easy to fix. Ordinarily, type errors occur
 while we are checking the source file in question, so we have plenty of
 SrcSpans from the parsed syntax tree to attach error to good source
 locations.

 However, a signature merge works differently: in general, we will have
 some interface files from our dependencies which we would like to merge
 together. If there is an incompatibility, there is NO local source
 declaration that we can get a SrcSpan for. So, what should we report in
 this case? One possibility is that we want to report the location of the
 source file *that produced the interface file.* But that would involve
 saving file paths in interface files, which seems quite distasteful to me,
 especially since the source location might be in a temporary directory, in
 which case you just broke determinism.  So... I am not sure what to do
 here!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13266>
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