[GHC] #13061: Incorrect constraints given single flexible undecidable instance

[GHC]

#13061: Incorrect constraints given single flexible undecidable instance
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        Reporter:  JCarr             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:  Constraint,
      Resolution:                    |  UndecidableInstances,
                                     |  FlexibleInstances
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by JCarr:

@@ -14,1 +14,3 @@
- f will have type Eq a => a -> a -> Bool
+ f will have type Eq a => a -> a -> Bool.
+ A function of type A a => ... will always be rejected unless Eq is also
+ included.
@@ -21,3 +23,1 @@
- Then f will have the type A a => a -> a -> Bool.
- A function of type A a => ... will always be rejected unless Eq is also
- included
+ Then f will have the correct type A a => a -> a -> Bool.

New description:

 If a class has an instance in the form F a => G a in its file, and no
 other instances, then the functions in G a will have the constraint F a,
 rather than G a.

 Example file
 {{{#!hs
 {-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
 class A a where
     f :: a -> a
 instance {-# OVERLAPPABLE #-} Eq a => A a where
     f = id
 }}}

 f will have type Eq a => a -> a -> Bool.
 A function of type A a => ... will always be rejected unless Eq is also
 included.

 If we add:
 {{{#!hs
 instance A Int where
     f = id
 }}}
 Then f will have the correct type A a => a -> a -> Bool.

--

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13061#comment:2>
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