## #1251 closed bug (fixed)

# GADTs with newtype deriving can crash GHCi / cause Core Lint to fail

Reported by: | Owned by: | ||
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Priority: | normal | Milestone: | 6.8.1 |

Component: | Compiler | Version: | 6.6 |

Keywords: | Cc: | ||

Operating System: | Linux | Architecture: | x86 |

Type of failure: | None/Unknown | Test Case: | gadt/CasePrune |

Blocked By: | Blocking: | ||

Related Tickets: | Differential Rev(s): | ||

Wiki Page: |

### Description

In my attempts to prove that GHC was generating errors on valid code, I have stumbled on a way to crash six versions of GHC, both F and FC, and in one case passing Core Lint!

stefan@stefans:/tmp$ V=6.4.2 ; ghc-$V -V ; ghc-$V -e 'seq icA ()' A.hs ; ghc-$V -dcore-lint -e 'seq icA ()' A.hs The Glorious Glasgow Haskell Compilation System, version 6.4.2 *** Exception: Segmentation fault *** Exception: Segmentation fault stefan@stefans:/tmp$ V=6.6 ; ghc-$V -V ; ghc-$V -e 'seq icA ()' A.hs ; ghc-$V -dcore-lint -e 'seq icA ()' A.hs The Glorious Glasgow Haskell Compilation System, version 6.6 mkCase1: null alts tpl_X6{v} [lid] main:A.icA{v rFP} [gid] Segmentation fault mkCase1: null alts tpl_X6{v} [lid] main:A.icA{v rFP} [gid] ghc-6.6: panic! (the 'impossible' happened) (GHC version 6.6 for i386-unknown-linux): compileExpr <no location info>: In the expression: base:GHC.Err.error{(w) v 06} [gid] @ base:GHC.Base.(){(w) tc 40} "Impossible alternative" Argument value doesn't match argument type: Fun type: [base:GHC.Base.Char{(w) tc 3o}] -> base:GHC.Base.(){(w) tc 40} Arg type: base:GHC.Prim.Addr#{(w) tc 33} Arg: "Impossible alternative" Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug stefan@stefans:/tmp$ V=6.7 ; ghc-$V -V ; ghc-$V -e 'seq icA ()' A.hs ; ghc-$V -dcore-lint -e 'seq icA ()' A.hs The Glorious Glasgow Haskell Compilation System, version 6.7 mkCase1: null alts tpl_B1{v} [lid] main:A.icA{v rq2} [gid] Segmentation fault mkCase1: null alts tpl_B1{v} [lid] main:A.icA{v rq2} [gid] ghc-6.7: panic! (the 'impossible' happened) (GHC version 6.7 for i386-unknown-linux): compileExpr <no location info>: In the expression: base:GHC.Err.error{(w) v 06} [gid] @ base:GHC.Base.(){(w) tc 40} "Impossible alternative" Argument value doesn't match argument type: Fun type: [base:GHC.Base.Char{(w) tc 3o}] -> base:GHC.Base.(){(w) tc 40} Arg type: base:GHC.Prim.Addr#{(w) tc 33} Arg: "Impossible alternative" Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug stefan@stefans:/tmp$ V=6.7.20070213 ; ghc-$V -V ; ghc-$V -e 'seq icA ()' A.hs ; ghc-$V -dcore-lint -e 'seq icA ()' A.hs The Glorious Glasgow Haskell Compilation System, version 6.7.20070213 mkCase: null alts tpl_B1{v} [lid] main:A.icA{v rsd} [gid] Segmentation fault mkCase: null alts tpl_B1{v} [lid] main:A.icA{v rsd} [gid] ghc-6.7.20070213: panic! (the 'impossible' happened) (GHC version 6.7.20070213 for i386-unknown-linux): compileExpr <no location info>: In the expression: base:GHC.Err.error{(w) v 06} [gid] @ base:GHC.Base.(){(w) tc 40} "Impossible alternative" Argument value doesn't match argument type: Fun type: [base:GHC.Base.Char{(w) tc 3o}] -> base:GHC.Base.(){(w) tc 40} Arg type: base:GHC.Prim.Addr#{(w) tc 33} Arg: "Impossible alternative" Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug stefan@stefans:/tmp$ V=6.7.20070223 ; ghc-$V -V ; ghc-$V -e 'seq icA ()' A.hs ; ghc-$V -dcore-lint -e 'seq icA ()' A.hs The Glorious Glasgow Haskell Compilation System, version 6.7.20070223 mkCase: null alts tpl_B1{v} [lid] main:A.icA{v rs9} [gid] Segmentation fault mkCase: null alts tpl_B1{v} [lid] main:A.icA{v rs9} [gid] ghc-6.7.20070223: panic! (the 'impossible' happened) (GHC version 6.7.20070223 for i386-unknown-linux): compileExpr <no location info>: In the expression: base:GHC.Err.error{(w) v 06} [gid] @ base:GHC.Base.(){(w) tc 40} "Impossible alternative" Argument value doesn't match argument type: Fun type: [base:GHC.Base.Char{(w) tc 3o}] -> base:GHC.Base.(){(w) tc 40} Arg type: base:GHC.Prim.Addr#{(w) tc 33} Arg: "Impossible alternative" Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug stefan@stefans:/tmp$ V=6.7.20070323 ; ghc-$V -V ; ghc-$V -e 'seq icA ()' A.hs ; ghc-$V -dcore-lint -e 'seq icA ()' A.hs The Glorious Glasgow Haskell Compilation System, version 6.7.20070323 mkCase: null alts tpl_B1{v} [lid] main:A.icA{v rs4} [gid] Segmentation fault mkCase: null alts tpl_B1{v} [lid] main:A.icA{v rs4} [gid] ghc-6.7.20070323: panic! (the 'impossible' happened) (GHC version 6.7.20070323 for i386-unknown-linux): compileExpr <no location info>: In the expression: base:GHC.Err.error{(w) v 06} [gid] @ base:GHC.Base.(){(w) tc 40} "Impossible alternative" Argument value doesn't match argument type: Fun type: [base:GHC.Base.Char{(w) tc 3o}] -> base:GHC.Base.(){(w) tc 40} Arg type: base:GHC.Prim.Addr#{(w) tc 33} Arg: "Impossible alternative" Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug stefan@stefans:/tmp$ cat A.hs {-# OPTIONS_GHC -fglasgow-exts #-} module A(II(..), icA) where data II a where II :: II Int class IC a where ic :: II a instance IC Int where ic = II newtype A = A Int deriving(IC) icA = (ic :: II A)

### Change History (3)

### comment:1 Changed 11 years ago by

Milestone: | → 6.6.2 |
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### comment:2 Changed 10 years ago by

Resolution: | → fixed |
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Status: | new → closed |

Test Case: | → gadt/CasePrune |

Interesting bug. Turns out that GHC's optimiser was being to eager about eliminating dead case branches. I've added a long comment to Unify.lhs, reproduced below. Meanwhile, the bug is fixed in HEAD; won't be fixed in 6.6

Meanwhile, you are not justified in assuming that there is no non-bottom value of type (II A). You can't construct one in normal Haskell, but you can with newtype-deriving as you showed, and (more generally) you can in GHC's intermediate lanuage FC. Use a data type you really want to be sure.

Thank you for a nice example. newtype deriving is more powerful than I thought!

Simon

Note [Pruning dead case alternatives] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider data T a where T1 :: T Int T2 :: T a newtype X = MkX Int newtype Y = MkY Char type family F a type instance F Bool = Int Now consider case x of { T1 -> e1; T2 -> e2 } The question before the house is this: if I know something about the type of x, can I prune away the T1 alternative? Suppose x::T Char. It's impossible to construct a (T Char) using T1, Answer = YES (clearly) Suppose x::T (F a), where 'a' is in scope. Then 'a' might be instantiated to 'Bool', in which case x::T Int, so ANSWER = NO (clearly) Suppose x::T X. Then *in Haskell* it's impossible to construct a (non-bottom) value of type (T X) using T1. But *in FC* it's quite possible. The newtype gives a coercion CoX :: X ~ Int So (T CoX) :: T X ~ T Int; hence (T1 `cast` sym (T CoX)) is a non-bottom value of type (T X) constructed with T1. Hence ANSWER = NO (surprisingly) Furthermore, this can even happen; see Trac #1251. GHC's newtype-deriving mechanism uses a cast, just as above, to move from one dictionary to another, in effect giving the programmer access to CoX. Finally, suppose x::T Y. Then *even in FC* we can't construct a non-bottom value of type (T Y) using T1. That's because we can get from Y to Char, but not to Int. Here's a related question. data Eq a b where EQ :: Eq a a Consider case x of { EQ -> ... } Suppose x::Eq Int Char. Is the alternative dead? Clearly yes. What about x::Eq Int a, in a context where we have evidence that a~Char. Then again the alternative is dead. Summary We are really doing a test for unsatisfiability of the type constraints implied by the match. And that is clearly, in general, a hard thing to do. However, since we are simply dropping dead code, a conservative test suffices. There is a continuum of tests, ranging from easy to hard, that drop more and more dead code. For now we implement a very simple test: type variables match anything, type functions (incl newtypes) match anything, and only distinct data types fail to match. We can elaborate later.

### comment:3 Changed 10 years ago by

Milestone: | 6.6.2 → 6.8.1 |
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Looks like a bug all right; thanks for the report!