Relax TyEq:N: allow out-of-scope newtype DataCon

The 'bad_newtype' assertion in GHC.Tc.Solver.Canonical.canEqCanLHSFinish
failed to account for the possibility that the newtype constructor
might not be in scope, in which case we don't provide any guarantees
about canonicalising away a newtype on the RHS of a representational
equality.

Fixes #21010

4 files changed, 49 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T21010.hs b/testsuite/tests/typecheck/should_compile/T21010.hs
new file mode 100644
index 0000000000..249d0f12c0
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T21010.hs
@@ -0,0 +1,12 @@
+{-# LANGUAGE Haskell2010 #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE TypeFamilies #-}
+
+module T21010 ( CBind(..) ) where
+import T21010A ( WrapMono, Constrained(Dom), withMonoCoercible )
+
+class CBind m where
+  (>>-) :: (Dom m a, Dom m b) => m a -> (a -> m b) -> m b
+
+instance CBind (WrapMono ()) where
+  (>>-) = withMonoCoercible undefined
diff --git a/testsuite/tests/typecheck/should_compile/T21010A.hs b/testsuite/tests/typecheck/should_compile/T21010A.hs
new file mode 100644
index 0000000000..234e1c72d7
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T21010A.hs
@@ -0,0 +1,15 @@
+{-# LANGUAGE Haskell2010 #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE TypeFamilies #-}
+
+module T21010A ( WrapMono, Constrained(..), withMonoCoercible ) where
+import T21010B ( WrapMono(..), withMonoCoercible )
+
+import Data.Kind ( Type, Constraint )
+
+class Constrained (f :: Type -> Type) where
+  type Dom f (a :: Type) :: Constraint
+
+instance Constrained (WrapMono mono) where
+  type Dom (WrapMono mono) b = b ~ mono
diff --git a/testsuite/tests/typecheck/should_compile/T21010B.hs b/testsuite/tests/typecheck/should_compile/T21010B.hs
new file mode 100644
index 0000000000..2a19a7e7e7
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T21010B.hs
@@ -0,0 +1,21 @@
+{-# LANGUAGE Haskell2010 #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE RoleAnnotations #-}
+{-# LANGUAGE TypeFamilies #-}
+
+module T21010B where
+import Data.Coerce ( Coercible)
+import Data.Kind   ( Constraint, Type )
+
+newtype WrapFunctor f (a :: Type) = WrapFunctor {runFunctor :: f a}
+
+type role WrapMono representational phantom
+newtype WrapMono mono b = WrapMono mono
+
+withMonoCoercible
+  :: (Coercible (WrapMono mono other) mono => r)
+  -> r
+withMonoCoercible = \x -> x
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index ef13910c41..39878e3ce6 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -810,3 +810,4 @@ test('StaticPtrTypeFamily', normal, compile, [''])
 test('T20946', normal, compile, [''])
 test('T20996', normal, compile, [''])
 test('T20732', normal, compile, [''])
+test('T21010', [extra_files(['T21010A.hs', 'T21010B.hs'])], multimod_compile, ['T21010.hs', '-v0'])