[GHC] #11282: Error warns about non-injectivity of injective type family

[GHC]

#11282: Error warns about non-injectivity of injective type family
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.11
      Resolution:  duplicate         |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14369            |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * status:  new => closed
 * resolution:   => duplicate
 * related:   => #14369


Comment:

 Closing as a duplicate of #14369. After commit
 8846a7fdcf2060dd37e66b4d1f89bd8fdfad4620 (which fixed #14369), the error
 message no longer claims that `F` may not be injective:

 {{{
 $ /opt/ghc/8.4.3/bin/ghc Bug.hs -XTypeFamilyDependencies
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:10:9: error:
     • Could not deduce: F a ~ F a0
       from the context: G a0
         bound by the type signature for:
                    meth :: G a0 => F a0
         at Bug.hs:10:9-26
       The type variable ‘a0’ is ambiguous
     • In the ambiguity check for ‘meth’
       To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
       In the type signature: meth :: (G a => F a) -> ()
    |
 10 | meth :: (G a => F a) -> ()
    |         ^^^^^^^^^^^^^^^^^^
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11282#comment:6>
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