#4528: stand-alone deriving sometimes fails for GADTs
-------------------------------+--------------------------------------------
Reporter: nestra | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 7.0.1 | Keywords: deriving mechanism, GADTs
Testcase: | Blockedby:
Os: Linux | Blocking:
Architecture: x86_64 (amd64) | Failure: Compile-time performance bug
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Consider the following module
{{{
{-# LANGUAGE GADTs, StandaloneDeriving #-}
module Foo where
data Foo a where
A, B :: Foo Int
deriving instance Enum (Foo a)
}}}
Loading it into GHCi causes the following message:
{{{
ghc: panic! (the 'impossible' happened)
(GHC version 7.0.1 for x86_64-unknown-linux):
maybe_is_tagToEnum_call.extract_constr_Ids
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
}}}
In the case of Bounded instead of Enum in the original code, it does not
work either but the error message is more "even-tempered":
{{{
Foo.hs:7:1:
Couldn't match type `a' with `Int'
`a' is a rigid type variable bound by
the instance declaration at Foo.hs:7:32
In the expression: A
In an equation for `minBound': minBound = A
When typechecking the code for `minBound'
in a standalone derived instance for `Bounded (Foo a)':
To see the code I am typechecking, use -ddump-deriv
In the instance declaration for `Bounded (Foo a)'
Foo.hs:7:1:
Couldn't match type `a' with `Int'
`a' is a rigid type variable bound by
the instance declaration at Foo.hs:7:32
In the expression: B
In an equation for `maxBound': maxBound = B
When typechecking the code for `maxBound'
in a standalone derived instance for `Bounded (Foo a)':
To see the code I am typechecking, use -ddump-deriv
In the instance declaration for `Bounded (Foo a)'
Failed, modules loaded: none.
}}}
I suspect that this is also a bug, as it would be perfectly reasonable to
have minBound and maxBound for a type such as Foo Int.
A similar deriving fails also for Read class, among those I tried.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4528>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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