[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 request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1
  (Parser)                           |
      Resolution:                    |             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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Comment (by goldfire):

 I'm not sure why you think that `type` should introduce a ''pattern''
 synonym (which, indeed, must mention all its variables on the right-hand
 side). It just introduces a (not necessarily injective) synonym.

 It's true that `-XTypeSynonymInstances` allows you to use a type synonym
 in an instance head. This might suggest the pattern-like behavior you
 seek. But this works via the expansion of the synonym, meaning that a
 program with both `instance C (Silly Int Bool)` and `instance C (Silly
 Char Bool)` will be rightly rejected.

 Note that this behavior is not new.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15687#comment:1>
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