[GHC] #15828: Type family equation foralls allow strange re-quantification of class-bound type variables

[GHC]

#15828: Type family equation foralls allow strange re-quantification of class-bound
type variables
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.7
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5283
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by mayac):

 @goldfire do you think it would be worth including in this fix a comment
 to the type signature of `bindLHsTyVarBndrs` mentioning that the `mb_cls`
 argument should usually be `Nothing`? For example:
 {{{
  bindLHsTyVarBndrs :: HsDocContext
                    -> Maybe SDoc            -- Just d => check for unused
 tvs
                                             --   d is a phrase like "in
 the type ..."
                    -> Maybe a               -- Just _  => an associated
 type decl
 +                                           -- (should be Nothing unless
 in bindHsQTyVars)
                    -> [LHsTyVarBndr GhcPs]  -- User-written tyvars
                    -> ([LHsTyVarBndr GhcRn] -> RnM (b, FreeVars))
                    -> RnM (b, FreeVars)
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15828#comment:5>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler