Re: #15918: mkCastTy sometimes drops insoluble (Type ~ Constraint) coercions

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#15918: mkCastTy sometimes drops insoluble (Type ~ Constraint) coercions
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  ⊥
       Component:  Compiler          |              Version:  8.6.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  quantified-
                                     |  constraints/T15918
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |  https://gitlab.haskell.org/ghc/ghc/merge_requests/489
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Comment (by Marge Bot <ben+marge-bot@…>):

 In [changeset:"db039a4a10fc8fa9e03e6781d1c0dc33151beda6/ghc"
 db039a4a/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="db039a4a10fc8fa9e03e6781d1c0dc33151beda6"
 Add regression test for #15918

 The test case in #15918 no longer triggers an `ASSERT` failure on
 GHC HEAD, likely due to commit
 682783828275cca5fd8bf5be5b52054c75e0e22c (`Make a smart mkAppTyM`).
 This patch adds a regression test for #15918 to finally put it to
 rest.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15918#comment:18>