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A user of one of my libraries is reporting that it fails with the message <<loop>> when compiled in GHC 7.2.1. The error is coming from withMVar, and is reproducible.

Unfortunately, the problem is *very* tricky to reproduce. Attempting to debug it (such as adding print statements, re-ordering operations, or removing unrelated code) will cause it to vanish. Therefore, the test case I have attached is a full Cabal project with build instructions. Nearly every line of code is required, and changing or removing them hides the error again.

Changes which can hide the problem:

• Altering or removing the startup messages (even the version number).
• Removing unused attributes from types.
• Removing unused error checking.
• Using _ <- char 'a' instead of void (char 'a').
• Moving a function from one module to another.
• Simplifying loops which only run over one element.
• Building it all at once with --make, and not a separate cabal-dev install step.
• Building 'Main' with -O2.