diff options
| -rw-r--r-- | testsuite/tests/warnings/should_compile/T22151.hs | 16 | ||||
| -rw-r--r-- | testsuite/tests/warnings/should_compile/all.T | 1 |
2 files changed, 17 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 ()) diff --git a/testsuite/tests/warnings/should_compile/all.T b/testsuite/tests/warnings/should_compile/all.T index 3139a042e2..5a799ab885 100644 --- a/testsuite/tests/warnings/should_compile/all.T +++ b/testsuite/tests/warnings/should_compile/all.T @@ -53,4 +53,5 @@ test('DerivingTypeable', normal, compile, ['-Wderiving-typeable']) test('T18862a', normal, compile, ['']) test('T18862b', normal, compile, ['']) test('T20312', normal, compile,['-Wall']) +test('T22151', normal, compile, ['-Wredundant-constraints']) test('T22759', normal, compile, ['']) |