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rebindable-clash-warning-fix : correct warning logic

Shayne Fletcher requested to merge shayne-fletcher-da/ghc:rebindable-clash-warning-fix into master

A warning of the form "The fallible pattern ... is used together with -XRebindableSyntax. If this is intentional compile with -Wno-missing-monad-fail-instances" is colloquially called a "rebindable clash" warning.

If the rebindable syntax and monad fail desugaring language extensions are enabled, rebindable clash warnings are issued irrespective of the setting of -Wmissing-monad-fail-instances flag. This patch fixes that. Before this patch there was no way to suppress these warnings.

We expect:

  • If rebindable syntax is enabled and monad fail desugaring is enabled and the flag -Wmissing-monadfail-instances is in effect, rebindable clash warnings should be displayed;
  • If rebindable syntax is enabled and monad fail desugaring is enabled and the flag -Wno-missing-monadfail-instances is in effect, rebindable clash warnings should not be displayed;
  • If rebindable syntax is enabled and monad fail desugaring is not enabled then rebindable clash warnings should not be displayed.
  • If rebindable syntax is not enabled then rebindable clash warnings should not be displayed.

Put another way, we expect rebindable clash warnings iff rebindable syntax is enabled and monad fail desugaring is enabled and the -Wmissing-monad-fail-instances flag is set.

Program for testing:

{-# LANGUAGE RebindableSyntax, MonadFailDesugaring #-}
{-# OPTIONS_GHC -Wno-missing-monadfail-instances #-}

module Main where

import Prelude

catMaybes xs = do
    Just x <- xs
    return x

main = print 1

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