Renamer hangs when checking type family consistency

[gmainland]

checkFamInstConsistency goes into and endless loop in some situations when called from tcRnImports. I have attached a tarball containing a boiled-down example consisting of three .hs files and a shell script. The hang only happens when compiling the three files (in dependency order) manually with -c; the hang does not happen with --make. So it looks like importing a module is not loading something that checkFamInstConsistency is assuming is there.

[gmainland]

Test case exhibiting the bug

[gmainland]

This is with revision 69570789166052c90a335dadbfaa8cfc1ad84330 (HEAD as of the time of the bug report).

[simonpj]

Fixed by

Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Wed Mar 28 09:56:14 2012 +0100

    Add a crucial forkM on the superclass context of IfaceClass in tcIfaceDecl
    
    The absence of this was causing a loop when typechecking an interface
    where the superclass context mentioned an associated type
       class C (T a) => D a where
         data T a
    
    Fixes Trac #5970

 compiler/iface/BuildTyCl.lhs   |    7 +++----
 compiler/iface/LoadIface.lhs   |    2 +-
 compiler/iface/TcIface.lhs     |   36 +++++++++++++++++++++++++-----------
 compiler/typecheck/FamInst.lhs |    3 +++
 4 files changed, 32 insertions(+), 16 deletions(-)

The comment is wrong when it mentions #5970; the patch fixes this ticket #5955.