[ ghc-Bugs-1225368 ] Typechecker loop and stack overflow in 6.4

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Bugs item #1225368, was opened at 2005-06-22 08:29
Message generated for change (Settings changed) made by simonpj
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Category: Compiler (Type checker)
Group: None
>Status: Closed
>Resolution: Fixed
Priority: 5
Submitted By: Einar Karttunen (ekarttun)
Assigned to: Nobody/Anonymous (nobody)
Summary: Typechecker loop and stack overflow in 6.4

Initial Comment:
The following seems to trigger an loop in the typechecker,
which does from stack overflow after a while.
The example is quite pointless but shows
the bug.

newtype PRef a = PRef a
type Drop1 a = a
class Ref a r | a -> r  where readRef :: a -> r
instance Ref (PRef a) (Drop1 a) where readRef (PRef v) = v

Here is a dump from the test (bug.hs contains the above code):
e at yui ~/work/pcl/hpclc $ ghci -v -fglasgow-exts bug.hs 
   ___         ___ _
  / _ \ /\  /\/ __(_)
 / /_\// /_/ / /  | |      GHC Interactive, version 6.4, for Haskell 98.
/ /_\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Using package config file: /usr/lib/ghc-6.4/package.conf
Using package config file: /home/e/.ghc/i386-linux-6.4/package.conf
Hsc static flags: -static
*** Parser:
*** Desugar:
*** Simplify:
*** CorePrep:
*** ByteCodeGen:
Loading package base-1.0 ... linking ... done.
*** Parser:
*** Desugar:
*** Simplify:
*** CorePrep:
*** ByteCodeGen:
*** Chasing dependencies:
unload: retaining objs []
unload: retaining bcos []
Stable modules:
unload: retaining objs []
unload: retaining bcos []
*** Compiling Main             ( bug.hs, interpreted ):
compile: input file bug.hs
*** Checking old interface for Main:
Compiling Main             ( bug.hs, interpreted )
*** Parser:
*** Renamer/typechecker:
*** Exception: stack overflow


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Comment By: Simon Peyton Jones (simonpj)
Date: 2005-06-28 09:36

Message:
Logged In: YES 
user_id=50165

Excellent point.  Now fixed.

tc195.hs tests this case.

Thank you for the report

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You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=108032&aid=1225368&group_id=8032