stage2 build fails

Simon Peyton Jones simonpj at
Mon Jan 27 11:42:20 UTC 2020

It would be good to know how to fix this.  It's blocking my builds.
For some reason it doesn't seem to kill CI


From: Simon Peyton Jones
Sent: 25 January 2020 20:26
To: ghc-devs <ghc-devs at>
Subject: stage2 build fails

I'm getting this with "sh validate -legacy"

compiler/main/DynFlags.hs:1344:15: error: [-Woverlapping-patterns, -Werror=overlapping-patterns]

    Pattern match is redundant

    In an equation for 'settings': settings s | otherwise = ...


1344 |             | otherwise = panic $ "Invalid cfg parameters." ++ exampleString

     |               ^^^^^^^^^
This is when compiling the stage-2 compiler.  There's an ifdef in DynFlags thus

#if __GLASGOW_HASKELL__ <= 810

            | otherwise = panic $ "Invalid cfg parameters." ++ exampleString

but somehow it's not triggering for the stage2 compiler.
Any ideas?  It's blocking a full build.
This #ifdef was added in 8038cbd96f4, when GHC became better at reporting redundant code.

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