[GHC] #16059: checkValidType is defeated by a type synonym

[GHC]

#16059: checkValidType is defeated by a type synonym
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.7
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Huh.  Yes, I suppose hat `Foo` is acceptable in some usage contexts (e.g.
 as a quantified constraint) but not others (in a kind). So we can't
 validity-check `Foo` on its own.

 So I suppose we should always validity-check the expansion, tiresome
 though that is.

 Would you like to do that?

 The `synIsTau` field of a `SynonymTyCon` might allow us to short-circuit
 the common case where the RHS is totally vanilla.  (To do so we'd need to
 ensure that `synIsTau` is false if there is a `=>` in the RHS, not just a
 forall.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16059#comment:2>
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