[GHC] #11698: GHC's tct_closed flag is not being set correctly

[GHC]

#11698: GHC's tct_closed flag is not being set correctly
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           Reporter:  simonpj        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  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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 The `Note [Bindings with closed types]` in `TcEnv` describes when a
 binding is "closed", which in turn affects generalisation.  (See the Note
 for details.)

 But the implementation doesn't follow the Note.  Instead it uses this
 function in `TcEnv`:
 {{{
 isClosedLetBndr :: Id -> TopLevelFlag
 -- See Note [Bindings with closed types] in TcRnTypes
 -- Note that we decided if a let-bound variable is closed by
 -- looking at its type, which is slightly more liberal, and a whole
 -- lot easier to implement, than looking at its free variables
 isClosedLetBndr id
   | isEmptyVarSet (tyCoVarsOfType (idType id)) = TopLevel
   | otherwise                                  = NotTopLevel
 }}}
 It may be easier but it's also wrong.  Consider
 {{{
 f x = ( let g y = x+y in ...
       , x::Int)
 }}}
 Is `g` closed (which affects how definitions in `...` are generalised)?
 Well if we typecheck the second element of the tuple first, we may "know"
 that `x::Int` by the time we are inferring a type for `g`, conclude that
 `g` has no free type variables, and say that it is closed.

 But if we do the `x::Int` part second, so while type checking the `let` we
 think that `x::alpha`, then we'll say that `g` is open.

 This looks nasty.  I think we should do it the way the `Note` says.

 Thanks to Facundo for pointing this out. Not urgent, but plainly wrong.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11698>
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