[GHC] #15368: Type families, holes and -fdefer-type-errors may cause 'opt_univ fell into a hole' panic

[GHC]

#15368: Type families, holes and -fdefer-type-errors may cause 'opt_univ fell into
a hole' panic
-------------------------------------+-------------------------------------
        Reporter:  dramforever       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Compile-time      |            Test Case:
  crash or panic                     |  typecheck/should_compile/T15368
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by osa1):

 I believe this is fixed in 8.6 beta already so we should merge Phab:D4958
 and close this:

 {{{
 haskell $ ghc MorePanic.hs -fdefer-type-errors
 [1 of 1] Compiling MorePanic        ( MorePanic.hs, MorePanic.o )

 MorePanic.hs:9:15: warning: [-Wtyped-holes]
     • Found hole: _ :: (F a b, F a0 b0)
       Where: ‘a0’ is an ambiguous type variable
              ‘b0’ is an ambiguous type variable
              ‘a’, ‘b’ are rigid type variables bound by
                the type signature for:
                  trigger :: forall a b. a -> b -> (F a b, F b a)
                at MorePanic.hs:8:1-35
     • In the first argument of ‘transitive’, namely ‘_’
       In the expression: _ `transitive` trigger _ _
       In an equation for ‘trigger’:
           trigger _ _ = _ `transitive` trigger _ _
     • Relevant bindings include
         trigger :: a -> b -> (F a b, F b a) (bound at MorePanic.hs:9:1)
   |
 9 | trigger _ _ = _ `transitive` trigger _ _
   |               ^

 MorePanic.hs:9:15: warning: [-Wdeferred-type-errors]
     • Couldn't match type ‘F b a’ with ‘F b0 a0’
       Expected type: (F a b, F b a)
         Actual type: (F a b, F b0 a0)
       NB: ‘F’ is a non-injective type family
       The type variables ‘b0’, ‘a0’ are ambiguous
     • In the expression: _ `transitive` trigger _ _
       In an equation for ‘trigger’:
           trigger _ _ = _ `transitive` trigger _ _
     • Relevant bindings include
         trigger :: a -> b -> (F a b, F b a) (bound at MorePanic.hs:9:1)
   |
 9 | trigger _ _ = _ `transitive` trigger _ _
   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^

 MorePanic.hs:9:38: warning: [-Wtyped-holes]
     • Found hole: _ :: a0
       Where: ‘a0’ is an ambiguous type variable
     • In the first argument of ‘trigger’, namely ‘_’
       In the second argument of ‘transitive’, namely ‘trigger _ _’
       In the expression: _ `transitive` trigger _ _
     • Relevant bindings include
         trigger :: a -> b -> (F a b, F b a) (bound at MorePanic.hs:9:1)
   |
 9 | trigger _ _ = _ `transitive` trigger _ _
   |                                      ^

 MorePanic.hs:9:40: warning: [-Wtyped-holes]
     • Found hole: _ :: b0
       Where: ‘b0’ is an ambiguous type variable
     • In the second argument of ‘trigger’, namely ‘_’
       In the second argument of ‘transitive’, namely ‘trigger _ _’
       In the expression: _ `transitive` trigger _ _
     • Relevant bindings include
         trigger :: a -> b -> (F a b, F b a) (bound at MorePanic.hs:9:1)
   |
 9 | trigger _ _ = _ `transitive` trigger _ _
   |                                        ^
 haskell $ ghc --version
 The Glorious Glasgow Haskell Compilation System, version 8.6.0.20180810
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15368#comment:5>
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