This one actually generates completely bogus Core which doesn't pass CoreLint.

This is actually a regression introduced by one of the recent patches. It used
to work!

- This test fails with the overhauled rewrite rules.  This is (judging from
  -ddump-tc-trace) not because the new rules implementation goes wrong, but
  because the old implementation left out some improvement and hence passed
  this test by accident.

This is better than a top-level clean() when using threads