7.10.1RC2 compile problem? ghc: internal error: PAP object entered!

[Herbert Valerio Riedel]

On 2015-01-31 at 14:34:35 +0100, George Colpitts wrote:
> Maybe this is something I shouldn't be doing, but I thought it was worth
> mentioning in case I have found a compiler bug.
> Should I file a bug for this?
>
> cabal install *--allow-newer=base*  accelerate
> ...
> [10 of 10] Compiling Data.Label.Base  ( src/Data/Label/Base.hs,
> dist/build/Data/Label/Base.o )
> ghc: internal error: PAP object entered!
>     (GHC version 7.10.0.20150123 for x86_64_apple_darwin)
>     Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

I suspect this has to do with `--allow-newer=base` allowing
template-haskell-2.9.0.0 to be re-installed (which then becomes a
build-dependency of `fclabels`). GHC 7.10, however, comes with
template-haskell-2.10.0.0

e.g. try the following in an empty sandbox:

  cabal install fclabels  --allow-newer=base --constraint 'template-haskell == 2.9.*'

(wheares a plain `cabal install fclabels` in a fresh sandbox should
succeed w/o panics with GHC 7.10)