[GHC] #15687: Type synonym unused binds no warning?

[GHC]

#15687: Type synonym unused binds no warning?
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           Reporter:  AntC           |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.6.1
  (Parser)                           |
           Keywords:                 |  Operating System:  Windows
       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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 I was taken aback this was accepted

 {{{#!hs
 type Silly a b = Maybe b

 x :: Silly Int Bool
 x = Just True
 y :: Silly Char Bool
 y = Just False

 z = x == y                    -- returns False

 }}}

 What's with the `a` in `Silly`'s decl? That's not a phantom type. It's
 ignored and thrown away. So `x` and `y` are the same type, and can be
 compare for equality.

 I expected a rule: all tyvars in the `type`'s head must appear on rhs. Or
 at least a warning there was something silly. I tried `-Wall, -fwarn-
 unused-binds`, `-Wunused-type-patterns`.

 I was just checking up on [https://github.com/ghc-proposals/ghc-
 proposals/pull/173#issuecomment-424980779 a remark] that type synonyms are
 at the type level like implicit bidirectional pattern synonyms. For those,
 all vars must appear on both sides.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15687>
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