[GHC] #11016: PartialTypeSignatures trigger bogus "unbound implicit parameter" error

25 Oct 2015

      #11016: PartialTypeSignatures trigger bogus "unbound implicit parameter" error
-------------------------------------+-------------------------------------
           Reporter:  gridaphobe     |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #10846
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 GHC seems to forget about in-scope implicit parameters when there's a hole
 in the type signature. Given

 {{{#!haskell
 {-# LANGUAGE ImplicitParams, PartialTypeSignatures #-}

 module Bad where

 f1 :: (?loc :: Int, _) => Int
 f1 = ?loc

 f2 :: (?loc :: Int) => _
 f2 = ?loc
 }}}

 GHC incorrectly reports the following errors.

 {{{
 % ghc -fforce-recomp Bad.hs
 [1 of 1] Compiling Bad              ( Bad.hs, Bad.o )

 Bad.hs:8:6:
     Unbound implicit parameter (?loc::Int)
       arising from a use of implicit parameter ‘?loc’
     In the expression: ?loc
     In an equation for ‘f1’: f1 = ?loc

 Bad.hs:11:1:
     Occurs check: cannot construct the infinite type:
       _ ~ (?loc::Int) => _
     When checking that ‘f2’ has the specified type
       f2 :: (?loc::Int) => _
     Probable cause: the inferred type is ambiguous

 Bad.hs:11:6:
     Unbound implicit parameter (?loc::_)
       arising from a use of implicit parameter ‘?loc’
     Relevant bindings include f2 :: _ (bound at Bad.hs:11:1)
     In the expression: ?loc
     In an equation for ‘f2’: f2 = ?loc
 }}}

 `?loc` is very clearly bound by both `f1` and `f2`'s signature. `f2`
 additionally reports what looks to me to be a bogus occurs-check error; we
 know from the theta that `?loc :: Int` so the hole should be solved for
 `Int`.

 I suspect this is all related to the fact that PartialTypeSignatures
 triggers `tcPolyInfer` instead of `tcPolyCheck`.

 I think this is the same underlying bug as #10846.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11016
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