Fix issue #18262 by zonking constraints after solving

Zonk residual constraints in checkForExistence to reveal user type
errors.

Previously when `:instances` was used with instances that have TypeError
constraints the result would look something like:

instance [safe] s0 => Err 'A -- Defined at ../Bug2.hs:8:10

whereas after zonking, `:instances` now sees the `TypeError` and
properly eliminates the constraint from the results.

4 files changed, 18 insertions, 0 deletions

diff --git a/testsuite/tests/ghci/T18262/T18262.hs b/testsuite/tests/ghci/T18262/T18262.hs
new file mode 100644
index 0000000000..1734de0179
--- /dev/null
+++ b/testsuite/tests/ghci/T18262/T18262.hs
@@ -0,0 +1,10 @@
+{-# LANGUAGE TypeFamilies, FlexibleInstances, DataKinds, UndecidableInstances #-}
+
+import GHC.TypeLits
+
+data C = A | B
+
+class Err (a :: C)
+
+instance (TypeError ('Text "uh-oh")) => Err 'A
+instance Err 'B
diff --git a/testsuite/tests/ghci/T18262/T18262.script b/testsuite/tests/ghci/T18262/T18262.script
new file mode 100644
index 0000000000..9b70743fad
--- /dev/null
+++ b/testsuite/tests/ghci/T18262/T18262.script
@@ -0,0 +1,6 @@
+:load T18262.hs
+:set -XDataKinds
+-- Should report no instances
+:instances 'A
+-- Should report an instance with no constraints
+:instances 'B
diff --git a/testsuite/tests/ghci/T18262/T18262.stdout b/testsuite/tests/ghci/T18262/T18262.stdout
new file mode 100644
index 0000000000..76d7e8a596
--- /dev/null
+++ b/testsuite/tests/ghci/T18262/T18262.stdout
@@ -0,0 +1 @@
+instance [safe] Err 'B -- Defined at T18262.hs:10:10
diff --git a/testsuite/tests/ghci/T18262/all.T b/testsuite/tests/ghci/T18262/all.T
new file mode 100644
index 0000000000..fe8e28fe6d
--- /dev/null
+++ b/testsuite/tests/ghci/T18262/all.T
@@ -0,0 +1 @@
+test('T18262', [extra_files(['T18262.hs'])], ghci_script, ['T18262.script'])