ghc 8.4.1 and trac 13930

Evan Laforge qdunkan at
Wed May 2 03:09:55 UTC 2018

I recently noticed that with -O1, ghc was optimizing some code

    if Trace.traceShowId "b" $ True
        then return $ Left $ Serialize.BadMagic (Serialize.magicBytes
magic) file_magic
        else first Serialize.UnserializeError <$> Exception.evaluate
(Serialize.decode rest)

to always evaluate the 'else' branch.  This is fixed in 8.4.2, so I'm
pretty sure it's

I assume there's no point trying to get a minimal reproduction, since
it's a known issue and fixed.  Still,
says "incorrectly optimised, resulting in space leaks".

Maybe it should instead say "incorrectly optimised, resulting in
taking the wrong branch in 'if' expressions"?  That's a bit more
alarming, and is a stronger "upgrade to 8.4.2" signal.

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