Add a regression test for #21323

This bug was fixed at some point between GHC 9.0 and GHC 9.2;
this patch simply adds a regression test.

2 files changed, 56 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T21323.hs b/testsuite/tests/typecheck/should_compile/T21323.hs
new file mode 100644
index 0000000000..aa7d07da91
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T21323.hs
@@ -0,0 +1,55 @@
+{-# Language ConstraintKinds          #-}
+{-# Language FlexibleContexts         #-}
+{-# Language FlexibleInstances        #-}
+{-# Language GADTs                    #-}
+{-# Language ImpredicativeTypes       #-}
+{-# Language InstanceSigs             #-}
+{-# Language MultiParamTypeClasses    #-}
+{-# Language PolyKinds                #-}
+{-# Language PolyKinds                #-}
+{-# Language PolyKinds                #-}
+{-# Language QuantifiedConstraints    #-}
+{-# Language ScopedTypeVariables      #-}
+{-# Language StandaloneKindSignatures #-}
+{-# Language TypeApplications         #-}
+{-# Language TypeFamilies             #-}
+
+module T21323 where
+
+import Data.Kind
+import Data.Coerce
+import Data.Type.Coercion
+import GHC.Generics
+
+data Dict cls where
+ Dict :: cls => Dict cls
+
+type    ViaRep :: Type -> (k -> Type) -> Type
+newtype a `ViaRep` rep = ViaRep a
+
+class    Coercible (Rep a x) (rep x) => AuxCoercible a rep x
+instance Coercible (Rep a x) (rep x) => AuxCoercible a rep x
+
+type ForallAuxCoercible a rep = forall x. AuxCoercible a rep x
+
+instance (Generic a, ForallAuxCoercible a rep) => Generic (ViaRep a rep) where
+ type Rep (ViaRep a rep) = rep
+
+ from :: forall x. ViaRep a rep -> rep x
+ from (ViaRep a) = co undefined (from @a @x a) where -- coerce (from @a @x a) where
+
+  co :: Coercion (Rep a x) (rep x) -> Rep a x -> rep x
+  co = undefined
+
+  c :: Dict (ForallAuxCoercible a rep)
+  c = Dict
+
+-- This caused a Core Lint error on GHC 9.0:
+--
+--    Out of scope: irred_a1LI :: cls_a1LH[tau:3]
+--                  [LclId]
+--    In the RHS of $cfrom_a1KZ :: forall a (rep :: * -> *) x.
+--                                 (Generic a, ForallAuxCoercible a rep) =>
+--                                 ViaRep a rep -> Rep (ViaRep a rep) x
+
+ to = undefined
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index f7b6cb82e7..a503d60b7c 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -822,3 +822,4 @@ test('T18406b', normal, compile, ['-ddump-tc -fprint-explicit-foralls -dsuppress
 test('T18529', normal, compile, ['-ddump-tc -fprint-explicit-foralls -dsuppress-uniques -fprint-typechecker-elaboration'])
 test('T21023', normal, compile, ['-ddump-types'])
 test('T21205', normal, compile, ['-O0'])
+test('T21323', normal, compile, [''])