[GHC] #12785: GHC panic, `tcTyVarDetails` is missing a case

[GHC]

#12785: GHC panic, `tcTyVarDetails` is missing a case
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        Reporter:  heisenbug         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:  yes
      Blocked By:                    |             Blocking:
 Related Tickets:  #12590            |  Differential Rev(s):  Phab:D2931
       Wiki Page:                    |
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Comment (by simonpj):

 OK I have worked it out.

 We use `tyCoVarsOfType`, `tyCoVarsOfCt` etc to extract the free type
 variables of a type or constraint.  We don't have functions `tyVarsOfType`
 etc; they would be simple to write, but we don't have them.
 That means that we may get unwanted coercion variables floating around in
 the result set.  This is annoying, and one alternative would be to add a
 family of functions returning tyvars.

 But returning the coercion variables (which appear in casts in types)
 actually  causes no trouble ''provided all the "is" functions return False
 for coercion variables''.

 For example,
 {{{
 promoteTyVar tclvl tv
   | isFloatedTouchableMetaTyVar tclvl tv
   = ...
   | otherwise
   = return ()
 }}}
 Here `tv` might be a coercion variable, but it'll be ignored provided
 `isFloatedTouchableMetaTyVar` ignores it.

 So I'm ok with revering the patch, but please can we add a big Note with
 those functions explaining why coercion variables can occur is happening?

 Incidentally, in `TcSimplify` we have this:
 {{{
            ; let meta_tvs = filter (isTyVar <&&> isMetaTyVar) free_tvs
 ...
                    -- The isTyVar needs to weed out coercion variables
 }}}
 Once `isMetaTyVar` filters out coercion variables, we can remove the
 `isTyVar` here.

 Reid: over to you.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12785#comment:11>
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