#5515: panic! (the 'impossible' happened) with evil self-referential instance
---------------------------------+------------------------------------------
Reporter: andersk | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Compile-time crash
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This bit of code escaped from my fingertips:
{{{
#!hs
{-# LANGUAGE ConstraintKinds, FlexibleInstances, TypeFamilies,
UndecidableInstances #-}
class ctx (Arg ctx) => Some ctx where type Arg ctx
instance ctx a => Some ctx where type Arg ctx = a
}}}
and crashed GHC master with:
{{{
ghc: panic! (the 'impossible' happened)
(GHC version 7.3.20110927 for x86_64-unknown-linux):
tcTyVarDetails a{tv aav} [tv]
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
}}}
This causes a similar crash without using ConstraintKinds:
{{{
#!hs
{-# LANGUAGE FlexibleContexts, FlexibleInstances, MultiParamTypeClasses,
TypeFamilies, UndecidableInstances #-}
class C f a
class C f (Arg f) => Some f where type Arg f
instance C f a => Some f where type Arg f = a
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5515>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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