compiling template haskell internal error: ... not in scope during type checking, but it passed the renamer

example code:

{-# LANGUAGE TemplateHaskell #-}

module THBug1 where

import Language.Haskell.TH

data HCons a b = HCons a b
data HNil = HNil

mhlt :: [Type] -> Q Type
mhlt xss = [t| $(foldThing xss)|]
  where
    foldThing (x:xs) = [t| HCons $x $(foldThing xs)|]
    foldThing [] = [t| HNil |]

compiling gives:

~$ ghc -c THBug1.hs 

THBug1.hs:13:38:
    GHC internal error: `foldThing' is not in scope during type checking, but it passed the renamer
    tcg_type_env of environment: [(rge, Type constructor `HNil'),
                                  (rgg, Data constructor `HNil'), (rgi, Type constructor `HCons'),
                                  (rgk, Data constructor `HCons'), (rgJ, Identifier `THBug1.HNil'),
                                  (rgM, Identifier `THBug1.HCons')]
    In the expression: foldThing xs
    In the Template Haskell quotation [t| HCons $x $(foldThing xs) |]
    In the expression: [t| HCons $x $(foldThing xs) |]

ghc used is the latest ghc 6.12.1 from debian experimental (package version 6.1.12-3, which is latest as of 28 Jan)

further details:

~$ uname -a
Linux debiannew 2.6.32-trunk-686-bigmem #1 SMP Sun Jan 10 07:12:17 UTC 2010 i686 GNU/Linux
~$ gcc -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.4.3-1' 
--with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ 
--prefix=/usr --enable-shared --enable-multiarch --enable-linker-build-id 
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext 
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 
--program-suffix=-4.4 --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug 
--enable-objc-gc --enable-targets=all --with-arch-32=i486 --with-tune=generic 
--enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.4.3 (Debian 4.4.3-1) 
~$ ghc -c -v -dcore-lint THBug1.hs 
Glasgow Haskell Compiler, Version 6.12.1, for Haskell 98, stage 2 booted by GHC version 6.12.1
Using binary package database: /usr/lib/ghc-6.12.1/package.conf.d/package.cache
Using binary package database: /home/jake/.ghc/i386-linux-6.12.1/package.conf.d/package.cache
hiding package QuickCheck-1.2.0.0 to avoid conflict with later version QuickCheck-2.1.0.3
hiding package base-3.0.3.2 to avoid conflict with later version base-4.2.0.0
hiding package parsec-2.1.0.1 to avoid conflict with later version parsec-3.0.1
wired-in package ghc-prim mapped to ghc-prim-0.2.0.0-3fbcc20c802efcd7c82089ec77d92990
wired-in package integer-gmp mapped to integer-gmp-0.2.0.0-fa82a0df93dc30b4a7c5654dd7c68cf4
wired-in package base mapped to base-4.2.0.0-73995e854f236dc2acd577d7c791221d
wired-in package rts mapped to builtin_rts
wired-in package haskell98 mapped to haskell98-1.0.1.1-0fdaf3b26bc38c43ce8371edf538dbf6
wired-in package template-haskell mapped to template-haskell-2.4.0.0-92d419f5a3bd03d1c021561d3b29c041
wired-in package dph-seq mapped to dph-seq-0.4.0-1f5167ea371010387123b68e975177b2
wired-in package dph-par mapped to dph-par-0.4.0-4e569f28e047d67d87266113526bc6ec
Hsc static flags: -static
Created temporary directory: /tmp/ghc19231_0
*** Checking old interface for main:THBug1:
*** Parser:
*** Renamer/typechecker:

THBug1.hs:13:38:
    GHC internal error: `foldThing' is not in scope during type checking, but it passed the renamer
    tcg_type_env of environment: [(rge, Type constructor `HNil'),
                                  (rgg, Data constructor `HNil'), (rgi, Type constructor `HCons'),
                                  (rgk, Data constructor `HCons'), (rgJ, Identifier `THBug1.HNil'),
                                  (rgM, Identifier `THBug1.HCons')]
    In the expression: foldThing xs
    In the Template Haskell quotation [t| HCons $x $(foldThing xs) |]
    In the expression: [t| HCons $x $(foldThing xs) |]
*** Deleting temp files:
Deleting: /tmp/ghc19231_0/ghc19231_0.s
Warning: deleting non-existent /tmp/ghc19231_0/ghc19231_0.s
*** Deleting temp dirs:
Deleting: /tmp/ghc19231_0

Trac metadata

Trac field	Value
Version	6.12.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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