testsuite: Add (still broken) testcase for #12447

Test Plan: Validate

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2703

GHC Trac Issues: #12447

testsuite/tests/ghci/scripts/T12447.script

+:set -XRankNTypes -XConstraintKinds -XTypeApplications
+
+import Data.Typeable
+
+class Deferrable p where deferEither :: proxy p -> (p => r) -> Either String r
+
+instance (Typeable a, Typeable b) => Deferrable (a ~ b) where deferEither = undefined
+
+:t deferEither @(_ ~ _)

testsuite/tests/ghci/scripts/T12447.stdout

+deferEither @(_ ~ _)
+  :: Deferrable (w2 ~ w1) =>
+     proxy (w2 ~ w1) -> (w2 ~ w1 => r) -> Either String r

testsuite/tests/ghci/scripts/all.T

@@ -266,3 +266,4 @@ test('T12091',
      ghci_script, ['T12091.script'])
 test('T12523', normal, ghci_script, ['T12523.script'])
 test('T12024', normal, ghci_script, ['T12024.script'])
+test('T12447', expect_broken(12447), ghci_script, ['T12447.script'])