Panic due 'instance Foo a => Bar a => Baz a' style declarations
class Foo a
instance Show a => Num a => Foo a where
foo = undefined
results in
ghc: panic! (the 'impossible' happened)
(GHC version 7.4.2 for x86_64-unknown-linux):
compiler/rename/RnSource.lhs:430:14-81: Irrefutable pattern failed for pattern Data.Maybe.Just (inst_tyvars,
_,
SrcLoc.L _ cls,
_)
It doesn't matter whether foo is declared in the class definition or not.
Things that work:
-
tuple-style context
-
not defining foo in the instance: Not crashing, but complaining about a malformed head, which might or might not be the right thing to do, considering
foo :: Show a => Num a => a
foo = undefined
Trac metadata
Trac field | Value |
---|---|
Version | 7.4.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |