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Diffstat (limited to 'testsuite/tests/warnings/should_compile/T22151.hs')
-rw-r--r-- | testsuite/tests/warnings/should_compile/T22151.hs | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/testsuite/tests/warnings/should_compile/T22151.hs b/testsuite/tests/warnings/should_compile/T22151.hs new file mode 100644 index 0000000000..aa30b07e5f --- /dev/null +++ b/testsuite/tests/warnings/should_compile/T22151.hs @@ -0,0 +1,16 @@ +{-# LANGUAGE UndecidableInstances #-} +module T22151 where + +import Control.Monad.IO.Class (MonadIO(liftIO)) + +class (Applicative m, Monad m) => C m where + m :: m () + +-- This should not emit a -Wredundant-constraints warning. This is because +-- GHC should not expand the superclasses of the Given constraint `MonadIO m` +-- given that it is not Paterson-smaller than the instance head `C m`. (See +-- Note [Recursive superclasses] in GHC.Tc.TyCl.Instance for more on what +-- "Paterson-smaller" means.) As a result, we must provide the `Applicative m` +-- and `Monad m` constraints explicitly. +instance (Applicative m, Monad m, MonadIO m) => C m where + m = liftIO (pure ()) |