[GHC] #11451: Inconsistent warnings for unused binders in type and instance declarations

[GHC]

#11451: Inconsistent warnings for unused binders in type and instance declarations
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1825
       Wiki Page:                    |
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Comment (by RyanGlScott <ryan.gl.scott@…>):

 In [changeset:"6817703b31840620cca8596ca62ed70633934972/ghc" 6817703/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="6817703b31840620cca8596ca62ed70633934972"
 Split off -Wunused-type-variables from -Wunused-matches

 Summary:
 Previously, `-Wunused-matches` would fire whenever it detected unused type
 variables in a type family or data family instance. This can be annoying
 for
 users who wish to use type variable names as documentation, as being
 `-Wall`-compliant would mean that they'd have to prefix many of their type
 variable names with underscores, making the documentation harder to read.

 To avoid this, a new warning `-Wunused-type-variables` was created that
 only
 encompasses unused variables in family instances. `-Wunused-matches`
 reverts
 back to its role of only warning on unused term-level pattern names.
 Unlike
 `-Wunused-matches`, `-Wunused-type-variables` is not implied by `-Wall`.

 Fixes #11451.

 Test Plan: ./validate

 Reviewers: goldfire, ekmett, austin, hvr, simonpj, bgamari

 Reviewed By: simonpj, bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D1825

 GHC Trac Issues: #11451
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11451#comment:36>
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