[GHC] #9911: Pattern synonyms with no signatures should yield warnings

[GHC]

#9911: Pattern synonyms with no signatures should yield warnings
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              Reporter:  cactus      |            Owner:  cactus
                  Type:  feature     |           Status:  new
  request                            |        Milestone:  7.10.1
              Priority:  normal      |          Version:  7.8.3
             Component:  Compiler    |         Keywords:  PatternSynonyms
  (Type checker)                     |     Architecture:  Unknown/Multiple
            Resolution:              |       Difficulty:  Unknown
      Operating System:              |       Blocked By:
  Unknown/Multiple                   |  Related Tickets:
       Type of failure:  Incorrect   |
  warning at compile-time            |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by cactus):

 The reason this doesnt Just Work(tm) is that the set of names that need
 signatures (`sig_names` in `TcRnDriver.tcTopSrcDecls`) is computed from
 the `HsBind Name`s on the input side (so for a pattern synonym `P`, the
 set would contain `P` itself), but then when checking against this set,
 the names from the typechecked bindings are used; so for a pattern synonym
 `P`, we try to look up `$mP` for the matcher and `$bP` for the builder.

 Another problem is that even if we would look up `P` (e.g. by changing
 matcher names to be the original pattern name without the added `$m`
 prefix), the warning would show the type of the matcher, instead of the
 pattern synonym type.

 So all in all, a real solution to this will require using the typechecked
 pattern synonym itself (a `PatSyn`) when doing the signature check, since
 that has the correct `Name` and the type.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9911#comment:1>
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