GHC 7.9 panics (simplifier ticks exhausted) on `half-0.2`

Compiling hackage:half-0.2 results in a GHC Panic:

$ ghc --version && ghc --print-project-git-commit-id
The Glorious Glasgow Haskell Compilation System, version 7.9.20141202
bf2d75417b5be7e8a79a26ee57a81e00682dabd4

$ cabal get half-0.2 && cd half-0.2/src/Numeric/
Unpacking to half-0.2/

$ ghc -Wall -O -fforce-recomp -c Half.hs 
ghc: panic! (the 'impossible' happened)
  (GHC version 7.9.20141202 for x86_64-unknown-linux):
	Simplifier ticks exhausted
  When trying UnfoldingDone lvl_s5yo
  To increase the limit, use -fsimpl-tick-factor=N (default 100)
  If you need to do this, let GHC HQ know, and what factor you needed
  To see detailed counts use -ddump-simpl-stats
  Total ticks: 125160

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

Trac metadata

Trac field	Value
Version	7.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ekmett
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

changed weight to 7

since this is actually a regression to 7.8, I'm increasing the priority to high

Trac metadata

Trac field	Value
Priority	normal → high

Attached file Half.hs ($1048).

src/Numeric/Half.hs from half-0.2 package

What factor works?

I've tried up to -fsimpl-tick-factor=100000 and it still fails:

$ ghc -Wall -O -fforce-recomp -fsimpl-tick-factor=100000 -c Half.hs 
ghc: panic! (the 'impossible' happened)
  (GHC version 7.9.20141202 for x86_64-unknown-linux):
	Simplifier ticks exhausted
  When trying UnfoldingDone lvl_s5yo
  To increase the limit, use -fsimpl-tick-factor=N (default 100)
  If you need to do this, let GHC HQ know, and what factor you needed
  To see detailed counts use -ddump-simpl-stats
  Total ticks: 125160000

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

does it make sense to try even higher?

changed milestone to %7.10.1

Supposedly fixed via fbb42b2e

Fixed by this (whose commit message contains a mis-typed ticket number)

commit fbb42b2ea42b6467135f26db47d9c296e7ad75a3
Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Sat Dec 13 22:47:01 2014 +0000

    Pattern-synonym matcher and builder Ids must be *LocalIds*
    
    This easy-to-make mistake meant that pattern-synonym matcher and
    builder Ids weren't being treated as locally defined by the simpplier.
    That meant that we never looked up them up in the environment, got an
    out-of-date unfolding, which made the Simplifier fall into an infinite
    loop.  This was the cause of Trac #98587, but it was quite tricky to
    find!
    
    In a separate patch I'll make Lint check for locally-bound GlobalIds,
    since they are always an error.

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed
Test case	→ patsyn/should_compile/T9857

added compiler crash label

added Phigh label