ghc 8.4.1 and trac 13930

[Evan Laforge]

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 https://ghc.haskell.org/trac/ghc/ticket/13930

I assume there's no point trying to get a minimal reproduction, since
it's a known issue and fixed.  Still,
https://downloads.haskell.org/~ghc/8.4.2/docs/html/users_guide/8.4.2-notes.html
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.