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Over-long lists in pattern match coverage check

I'm seeing a lot of this

WARNING: file compiler/utils/Util.hs, line 593
  Over-long elem in unionLists
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1294:29 in ghc:Outputable
        warnPprTrace, called at compiler/utils/Util.hs:593:20 in ghc:Util
        isIn, called at compiler/utils/ListSetOps.hs:64:5 in ghc:ListSetOps
        unionLists, called at compiler/GHC/HsToCore/PmCheck/Oracle.hs:940:36 in ghc:GHC.HsToCore.PmCheck.Oracle

E.g. when compiling Cabal.

Something is bad in Oracle.hs.

Edited by Simon Peyton Jones
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