[GHC] #11660: Remove Type pretty-printer in favor of IfaceType

[GHC]

#11660: Remove Type pretty-printer in favor of IfaceType
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        Reporter:  bgamari           |                Owner:
            Type:  task              |               Status:  patch
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12550, #12447,   |  Differential Rev(s):  Phab:D2528
  #11786, #11549, #12024, #12697,    |
  #12510                             |
       Wiki Page:                    |
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Comment (by bgamari):

 I've updated Phab:D2528 with the current state of my branch, which almost
 passes the testsuite. There 21 testsuite failures, nearly all of which are
 trivial changes in parentheses. In fact, I am quite tempted to accept the
 new output since it looks significantly more consistent than the previous
 output.

 However, there is one rather tricky testsuite failure that I'm a bit
 unsure how to handle. Namely, when we go to print an equality we look at
 the types involved to determine what operator to print. However, since we
 only have `IfaceType`s we can't see through type synonyms. This means that
 if we have an equality,

 {{{
 (a :: GHC.Types.*) ~ (b :: GHC.Types.⋆)
 }}}
 we will render it as a heterogeneous equality (with a kind annotation on
 each operand) since the kinds look different (although in reality are
 synonyms).

 The solution for this that I am currently leaning towards is to represent
 equalities explicitly in `IfaceType` and ensure that they carry their
 "flavor" for pretty-printing purposes.


 **Failing tests**: `TEST="TypeSkolEscape T10632 T7939 T2431 ghci059 T7525
 T7019a tcfail130 T9222 T8912 T8958 T12634 T2766 IPFail ContextStack2
 T11252 ClassOperator tcfail041 tcrun045 tcfail211 T10858 T5837"`

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11660#comment:17>
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