diff options
| -rw-r--r-- | compiler/typecheck/TcValidity.hs | 15 | ||||
| -rw-r--r-- | testsuite/tests/indexed-types/should_fail/T15172.hs | 11 | ||||
| -rw-r--r-- | testsuite/tests/indexed-types/should_fail/T15172.stderr | 5 | ||||
| -rw-r--r-- | testsuite/tests/indexed-types/should_fail/all.T | 1 | ||||
| -rw-r--r-- | testsuite/tests/typecheck/should_fail/tcfail214.stderr | 2 |
5 files changed, 32 insertions, 2 deletions
diff --git a/compiler/typecheck/TcValidity.hs b/compiler/typecheck/TcValidity.hs index bdda6cd455..35e6a956f3 100644 --- a/compiler/typecheck/TcValidity.hs +++ b/compiler/typecheck/TcValidity.hs @@ -1405,8 +1405,10 @@ checkInstTermination tys theta check2 pred pred_size | not (null bad_tvs) = addErrTc (noMoreMsg bad_tvs what) + | not (isTyFamFree pred) = addErrTc (nestedMsg what) | pred_size >= head_size = addErrTc (smallerMsg what) | otherwise = return () + -- isTyFamFree: see Note [Type families in instance contexts] where what = text "constraint" <+> quotes (ppr pred) bad_tvs = fvType pred \\ head_fvs @@ -1432,7 +1434,18 @@ undecidableMsg, constraintKindsMsg :: SDoc undecidableMsg = text "Use UndecidableInstances to permit this" constraintKindsMsg = text "Use ConstraintKinds to permit this" -{- +{- Note [Type families in instance contexts] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Are these OK? + type family F a + instance F a => C (Maybe [a]) where ... + intance C (F a) => C [[[a]]] where ... + +No: the type family in the instance head might blow up to an +arbitrarily large type, depending on how 'a' is instantiated. +So we require UndecidableInstances if we have a type family +in the instance head. Trac #15172. + Note [Associated type instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We allow this: diff --git a/testsuite/tests/indexed-types/should_fail/T15172.hs b/testsuite/tests/indexed-types/should_fail/T15172.hs new file mode 100644 index 0000000000..da7a8f80f2 --- /dev/null +++ b/testsuite/tests/indexed-types/should_fail/T15172.hs @@ -0,0 +1,11 @@ +{-# LANGUAGE TypeFamilies, ConstraintKinds, FlexibleInstances #-} +module ShouldFail where + +import GHC.Exts( Constraint ) + +type family F a :: Constraint + +class C a where + +-- Should be rejected because of the type family +instance (F a) => C [[a]] where diff --git a/testsuite/tests/indexed-types/should_fail/T15172.stderr b/testsuite/tests/indexed-types/should_fail/T15172.stderr new file mode 100644 index 0000000000..8c28c5148c --- /dev/null +++ b/testsuite/tests/indexed-types/should_fail/T15172.stderr @@ -0,0 +1,5 @@ + +T15172.hs:11:10: error: + • Illegal nested constraint ‘F a’ + (Use UndecidableInstances to permit this) + • In the instance declaration for ‘C [[a]]’ diff --git a/testsuite/tests/indexed-types/should_fail/all.T b/testsuite/tests/indexed-types/should_fail/all.T index 4d25d970b9..61025d6c92 100644 --- a/testsuite/tests/indexed-types/should_fail/all.T +++ b/testsuite/tests/indexed-types/should_fail/all.T @@ -141,3 +141,4 @@ test('T14033', normal, compile_fail, ['']) test('T14045a', normal, compile_fail, ['']) test('T14175', normal, compile_fail, ['']) test('T14369', normal, compile_fail, ['']) +test('T15172', normal, compile_fail, ['']) diff --git a/testsuite/tests/typecheck/should_fail/tcfail214.stderr b/testsuite/tests/typecheck/should_fail/tcfail214.stderr index c6a4387142..83fa344834 100644 --- a/testsuite/tests/typecheck/should_fail/tcfail214.stderr +++ b/testsuite/tests/typecheck/should_fail/tcfail214.stderr @@ -1,5 +1,5 @@ tcfail214.hs:9:10: error: - • The constraint ‘F a’ is no smaller than the instance head + • Illegal nested constraint ‘F a’ (Use UndecidableInstances to permit this) • In the instance declaration for ‘C [a]’ |