[GHC] #12201: Wrong instance selection with overlapping instance in a superclass (was: Wrong instance selection with overlapping instances and local bindings)

[GHC]

#12201: Wrong instance selection with overlapping instance in a superclass
-------------------------------------+-------------------------------------
        Reporter:  kanetw            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by kanetw:

@@ -1,2 +1,2 @@
- test and test2 should have the same type, but it seems like the non-{#-
- OVERLAPPING #-} instance is chosen prematurely.
+ test_foo and test_bar should have the same type, but it seems like the
+ non-{#- OVERLAPPING #-} instance is chosen prematurely.
@@ -16,3 +16,0 @@
- bar :: (A a, B a) => a
- bar = undefined
-
@@ -22,5 +19,5 @@
- {-
-   *Bug> :t test
-   test :: A (Foo s Bool) => s -> Int
- -}
- test = helper bar
+ foo :: (A a, B a) => a
+ foo = undefined
+
+ bar :: B a => a
+ bar = undefined
@@ -29,2 +26,2 @@
-   *Bug> :t test2
-   test2 :: s -> Int
+   *Bug> :t test_foo
+   test_foo :: A (Foo s Bool) => s -> Int
@@ -32,1 +29,7 @@
- test2 = let foo = bar in helper foo
+ test_foo = helper foo
+
+ {-
+   *Bug> :t test_bar
+   test_bar :: s -> Int
+ -}
+ test_bar = helper bar
@@ -37,4 +40,4 @@
- *Bug> :t test
- test :: A (Foo s Bool) => s -> Int
- *Bug> :t test2
- test2 :: A (Foo s Bool) => s -> Int
+ *Bug> :t test_foo
+ test_foo :: A (Foo s Bool) => s -> Int
+ *Bug> :t test_bar
+ test_bar :: A (Foo s Bool) => s -> Int

New description:

 test_foo and test_bar should have the same type, but it seems like the
 non-{#- OVERLAPPING #-} instance is chosen prematurely.
 {{{#!hs
 {-# LANGUAGE NoMonomorphismRestriction, FlexibleContexts,
 FlexibleInstances #-}
 module Bug where

 class A a where
 class A a => B a where

 data Foo s a
 instance {-# OVERLAPPING #-} A (Foo Int Bool)
 instance A (Foo s a)
 instance B (Foo s a)

 helper :: Foo s Bool -> s -> Int
 helper = undefined

 foo :: (A a, B a) => a
 foo = undefined

 bar :: B a => a
 bar = undefined

 {-
   *Bug> :t test_foo
   test_foo :: A (Foo s Bool) => s -> Int
 -}
 test_foo = helper foo

 {-
   *Bug> :t test_bar
   test_bar :: s -> Int
 -}
 test_bar = helper bar
 }}}

 Compare with 7.10.3:
 {{{
 *Bug> :t test_foo
 test_foo :: A (Foo s Bool) => s -> Int
 *Bug> :t test_bar
 test_bar :: A (Foo s Bool) => s -> Int
 }}}

--

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12201#comment:4>
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