Summary

The attached zip contains a small cabal package with a Win32 dependency. The problematic source file in question is Offending.hs, where the optimisation level was overridden.

Steps to reproduce

When setting the optimisation level in Offending.hs to -O1, and doing a full rebuild (running cabal clean if neccessary), the resulting program prints the following:

Sending mask: 0

Somehow, the second call to sendMessage in Offending.hs gets dropped.

Expected behavior

When setting the optimisation level in Offending.hs to -O0, the program prints the expected result:

Sending mask: 0
Recieved new mask: 2

Environment

• GHC version used: 8.10.1

bug.zip