[GHC] #12441: Conflicting definitions error does not print explicit quantifiers when necessary

[GHC]

#12441: Conflicting definitions error does not print explicit quantifiers when
necessary
-------------------------------------+-------------------------------------
        Reporter:  ezyang            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  newcomers
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):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by bgamari):

 * keywords:   => newcomers
 * failure:  None/Unknown => Incorrect warning at compile-time


@@ -1,1 +1,1 @@
- {{{
+ {{{#!hs
@@ -6,0 +6,1 @@
+
@@ -9,0 +10,1 @@
+

New description:

 {{{#!hs
 -- Y.hs-boot
 {-# LANGUAGE ScopedTypeVariables #-}
 module Y where
 f :: forall a b. (a, b)

 -- YY.hs
 module YY where
 import {-# SOURCE #-} Y

 -- Y.hs
 {-# LANGUAGE ScopedTypeVariables #-}
 module Y where
 import YY
 f :: forall b a. (a, b)
 f = undefined
 }}}

 I get the following unhelpful error:

 {{{
 ezyang at sabre:~$ ghc-8.0 --make Y.hs -fforce-recomp
 [1 of 3] Compiling Y[boot]          ( Y.hs-boot, Y.o-boot )
 [2 of 3] Compiling YY               ( YY.hs, YY.o )
 [3 of 3] Compiling Y                ( Y.hs, Y.o )

 Y.hs-boot:3:1: error:
     Identifier ‘f’ has conflicting definitions in the module
     and its hs-boot file
     Main module: f :: (a, b)
     Boot file:   f :: (a, b)
     The two types are different
 }}}

 Yes this example is purposely shooting itself in the foot, but in the wild
 I encountered an un-annotated type which inferred a different quantifier
 ordering than what I expected, and I subsequently spent a while puzzling
 over the error message. `-fprint-explicit-foralls` is a sufficient
 workaround.

--

Comment:

 I think this would be a reasonable thing for a newcomer to knock off. Just
 check whether the two types are equivalent up to quantifiers and render
 them with quantifiers if so.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12441#comment:1>
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