[GHC] #13512: GHC incorrectly warns that a variable used in a type application is unused

[GHC]

#13512: GHC incorrectly warns that a variable used in a type application is unused
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:                    |  TypeApplications
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Ah, you're right, it is in fact `rnValBindsRHS` that handles the renaming
 of function type signatures, not `rnValBindsLHS`. My mistake.

 But my point about there being a chicken-and-egg problem is still
 relevant, I believe. Here is the
 [http://git.haskell.org/ghc.git/blob/14457cf6a50f708eecece8f286f08687791d51f7:/compiler/rename/RnBinds.hs#l287
 current definition] of `rnValBindsRHS`:

 {{{#!hs
 rnValBindsRHS :: HsSigCtxt
               -> HsValBindsLR GhcRn GhcPs
               -> RnM (HsValBinds GhcRn, DefUses)
 rnValBindsRHS ctxt (ValBindsIn mbinds sigs)
   = do { (sigs', sig_fvs) <- renameSigs ctxt sigs
        ; binds_w_dus <- mapBagM (rnLBind (mkSigTvFn sigs')) mbinds
        ; let !(anal_binds, anal_dus) = depAnalBinds binds_w_dus
        ; let patsyn_fvs = foldr (unionNameSet . psb_fvs) emptyNameSet $
                           getPatSynBinds anal_binds
              valbind'_dus = anal_dus `plusDU` usesOnly sig_fvs
                                      `plusDU` usesOnly patsyn_fvs
         ; return (ValBindsOut anal_binds sigs', valbind'_dus) }
 rnValBindsRHS _ b = pprPanic "rnValBindsRHS" (ppr b)
 }}}

 Notice that the call to `renameSigs` (in which all `Unused quantified type
 variable` warnings are emitted) occurs before the call to `rnLBind` (which
 would handle, e.g., `SomeProxy (proxy @k @a)`). Moreover, this has to be
 the case, since the result of `renameSigs` is fed into `rnLBind`. This
 makes me believe that the `Unused quantified type variable` analysis needs
 to be moved out of `bindLHsTyVarBndrs` and somewhere that can take into
 account the results of `rnValBindsRHS`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13512#comment:6>
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