Fix #18071

Run the core linter on candidate instances to ensure they are
well-kinded.

Better handle quantified constraints by using a CtWanted to avoid
having unsolved constraints thrown away at the end by the solver.

4 files changed, 42 insertions, 0 deletions

diff --git a/testsuite/tests/ghci/T18071/T18071.hs b/testsuite/tests/ghci/T18071/T18071.hs
new file mode 100644
index 0000000000..1b0d53c337
--- /dev/null
+++ b/testsuite/tests/ghci/T18071/T18071.hs
@@ -0,0 +1,32 @@
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE QuantifiedConstraints #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE DataKinds #-}
+module Bug where
+
+import Data.Kind
+import Data.Proxy
+
+class MyShow a where
+  show :: a -> String
+
+class    (forall (z :: k). MyShow (Proxy z)) => MyShowProxy (k :: Type) where
+instance (forall (z :: k). MyShow (Proxy z)) => MyShowProxy (k :: Type)
+
+data T :: Type -> Type
+
+data U = A | B
+
+instance forall (z :: U). MyShow (Proxy (z :: U)) where
+  show _ = "U"
+
+data U2 = A2 | B2
+
+instance MyShow (Proxy A2) where
+  show _ = "A2"
+
+instance MyShow (Proxy B2) where
+  show _ = "B2"
+
diff --git a/testsuite/tests/ghci/T18071/T18071.script b/testsuite/tests/ghci/T18071/T18071.script
new file mode 100644
index 0000000000..1727f26acc
--- /dev/null
+++ b/testsuite/tests/ghci/T18071/T18071.script
@@ -0,0 +1,8 @@
+:set -Wno-simplifiable-class-constraints
+:load T18071.hs
+-- Should report no instances since it is ill-kinded for T
+:instances T
+-- U should report a match for ShowProxy
+:instances U
+-- U2 should not report a match for ShowProxy
+:instances U2
diff --git a/testsuite/tests/ghci/T18071/T18071.stdout b/testsuite/tests/ghci/T18071/T18071.stdout
new file mode 100644
index 0000000000..815aa1e245
--- /dev/null
+++ b/testsuite/tests/ghci/T18071/T18071.stdout
@@ -0,0 +1 @@
+instance [safe] MyShowProxy U -- Defined at T18071.hs:16:10
diff --git a/testsuite/tests/ghci/T18071/all.T b/testsuite/tests/ghci/T18071/all.T
new file mode 100644
index 0000000000..9eef5bd165
--- /dev/null
+++ b/testsuite/tests/ghci/T18071/all.T
@@ -0,0 +1 @@
+test('T18071', [extra_files(['T18071.hs'])], ghci_script, ['T18071.script'])