#9380: ghc generates seemingly incorrect code -------------------------------------+------------------------------------- Reporter: qnikst | Owner: Type: bug | Status: infoneeded Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: Incorrect | Blocked By: result at runtime | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by qnikst): There are few reasons for this report: 1. Verify that this is a really wrong behaviour 2. Make sure that fix will go into 7.8.4 if it will ever be 3. Understand if it worth adding a regression test for the HEAD, in order track that behaviour will be correct. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9380#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#9380: ghc generates seemingly incorrect code -------------------------------------+------------------------------------- Reporter: qnikst | Owner: Type: bug | Status: infoneeded Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: Incorrect | Blocked By: result at runtime | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by simonpj): I think I know what is going on. Consider `test0`: {{{ test0 = let s = cast A (SomeS (S 0)) in case viewV0 s of V0A{} -> putStrLn "test0 - A" V0B{} -> putStrLn "test0 - B" }}} Remember that `cast :: forall a. M -> SomeS -> S a`, so that `s` is a polymorphic value: `s :: forall (x::M). S x`. Now at the occurrence of `s` on the next line, GHC has to choose what type to instantiate `s` at; that is, what to instantiate `x` with. The calling context is no help, so GHC simply has to guess any old type, of kind `M`. It could randomly choose `A` or `B`, but that seems arbitrary. So it chooses `Any M`, where `Any :: forall k. k` is a built-in type family that has no equations. It's as if it was declared with {{{ type family Any :: k }}} So now `(viewV0 s)` has type `V0 (Any M)`. The trouble is that in GHC 7.8, `Any` was a ''data type'' not a type family. That was wrong for many reasons, but this ticket exposes a new one. In a `case` expression, GHC discards any patterns that can't possibly match the scrutinee. The two patterns in the case expression in `test0` have types `V0 A` and `V0 B`. Since `A` and `Any M` are different (if `Any` is a data type) GHC discarded that alternative, and the same for the other one. In HEAD, `Any` is a type family (see #9097), so GHC (rightly) does not know that `Any M` and `A` are distinct. So the alternative is not discarded. I have not followed through all the other cases, but I bet that it's all attributable to that one fix. I will add this as regression test, despite the unsavoury use of `unsafeCoerce`. I don't propose to fix 7.8.4. I think there were various reasons that `Any` was not made a type family earlier. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9380#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#9380: ghc generates seemingly incorrect code -------------------------------------+------------------------------------- Reporter: qnikst | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Resolution: fixed | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: Incorrect | Blocked By: result at runtime | Related Tickets: Test Case: gadt/T9380 | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by maxtaldykin): Here is a bit more concise test case (derived from the original one): {{{ {-# LANGUAGE GADTs, DataKinds, KindSignatures #-} import Unsafe.Coerce data X = A | B data Y (x :: X) where YA :: Y A YB :: Y B Yany :: String -> Y x view :: Y a -> Y b view = unsafeCoerce main :: IO () main = case view YA of YA -> putStrLn "YA" YB -> putStrLn "YB" Yany x -> putStrLn x }}} This will cause segmentation fault on 7.8.3 due to matching `YA` with `Yany` and trying to access `String` that does not exist. In HEAD it will print `YA`. But is it really intended behavior? My reasoning is that this is an example of unsafe using of `unsafeCoerce`. Due to `view`'s parametricity GHC can infer that all valid implementations (except _|_) are of the form {{{ view :: Y a -> Y b view = const (Yany "some string here") }}} And it is safe to optimize `main` by dropping `YA` and `YB` cases. But if we allow such usage of `unsafeCoerce` then it is no longer possible to optimize cases even if `view` absolutely safe. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9380#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#9380: ghc generates seemingly incorrect code -------------------------------------+------------------------------------- Reporter: qnikst | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Resolution: fixed | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: Incorrect | Blocked By: result at runtime | Related Tickets: Test Case: gadt/T9380 | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones ): In [changeset:"dc7d3c2d437b310d26b05033d1b34601e1914d00/ghc"]: {{{ #!CommitTicketReference repository="ghc" revision="dc7d3c2d437b310d26b05033d1b34601e1914d00" Test Trac #9380 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9380#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler