[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
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.7
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  rename/should_fail/T15828
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5283
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Replying to [comment:10 simonpj]:
 > What is 12eeb9?

 It's referring to commit 512eeb9bb9a81e915bfab25ca16bc87c62252064 (the
 leading "5" was simply dropped by mistake).

 > Quite true.  So what would we ''like'' to show here?  Once we decide
 that we can think how to achieve it.

 I was quite fond of the way it was printed at the time that I landed the
 patch:

 {{{
     • Type indexes must match class instance head
       Expected: T (Maybe a1) <tv>
         Actual: T (Maybe a) b
 }}}

 That is, using tidying to emphasize the difference between the two `a`s.

 Unfortunately, I didn't notice that your commit had changed this until I
 `validate`d the whole set of patches I was about to commit, at which point
 I just accepted the new output and moved on. (Upon further thought, I
 realized the new output was rather smelly, which is why I reopened this
 ticket.)

 > I'm quite keen on using "_" for the bits that don't matter

 Sure. I have no preference as to whether we display other type variables
 as `_`, `<tv>`, or something else.

 > but if needs must, instead of printing `(F t1 t2 t3)` with one call, we
 can make separate calls for `t1`, `t2`, `t3` etc, and that would eliminate
 the need to fake-up a tyvar "_" which you'll see in the code.

 I'm afraid I have no idea what you are referring to with this sentence.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15828#comment:11>
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