GHC panic (nameModule) on malformed deriving clause

[Philonous]

Trying to derive a classname starting with a lower case letter and wrapped in parenthesis will lead to a ghc panic instead of generating an error

minimal testcase:

module Foo where
data Foo = Foo deriving (abc)

output:

[1 of 1] Compiling Foo              ( scratch3.hs, scratch3.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 7.4.1 for x86_64-unknown-linux):
        nameModule abc{tv a9I}

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

[pcapriotti]

Thanks for the report. This seems to be fixed in HEAD.

[simonpj]

Indeed:

T5961.hs:2:26:
    Illegal deriving item `abc'
    In the data declaration for `Foo'