[GHC] #14131: Difference between newtype and newtype instance

[GHC]

#14131: Difference between newtype and newtype instance
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        Reporter:  Iceland_jack      |                Owner:  RyanGlScott
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #7938, #9574,     |  Differential Rev(s):  Phab:D3872
  #13985                             |
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Replying to [comment:13 simonpj]:
 > Really?  `reportFloatingKvs` seems to report variables mentioned in
 `typat_tvs`, which doesn't look like the ones on the RHS.   I wasn't
 expecting a test in `tcFamTyPats`, but rather in `tcTyFamInstEqn`.

 Ah, I think I managed to confuse you by naming that variable `typat_tvs`.
 Those are the type variables that are used in type patterns (visible and
 invisible) //or// the RHS (these are collected during renaming, see
 `rnFamInstDecl`). We discover in `tcFamTyPats` which variables are
 actually bound by type patterns, so if there are any used variables that
 aren't bound, then they're free-floating (and summarily rejected).

 I suppose I should rename `typat_tvs` to something like `fam_used_tvs` to
 avoid this confusion.

 > DO you have a test like the example in comment:8?

 Yes, see `testsuite/tests/polykinds/T13985.hs`. In the particular case of
 type family instances, it tests this:

 {{{#!hs
 type instance T = Proxy (Nothing :: Maybe a)
 }}}

 Which is rejected for having a free-floating `a`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14131#comment:14>
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