Add regression test for #22328

testsuite/tests/patsyn/should_compile/T22328.hs

+{-# LANGUAGE TypeApplications, PatternSynonyms, GADTs, ViewPatterns #-}
+
+module T22328 where
+
+import Data.Typeable
+
+data Gadt x y where
+  ExistentialInGadt :: Typeable a => a -> Gadt x x
+
+pattern CastGadt :: Typeable a => x ~ y => a -> Gadt x y
+pattern CastGadt a <- ExistentialInGadt (cast -> Just a)
+
+test :: Gadt i o -> Bool
+test gadt = case gadt of
+  CastGadt @Bool a -> a
+  _ -> False

testsuite/tests/patsyn/should_compile/all.T

@@ -84,3 +84,4 @@ test('T14630', normal, compile, ['-Wname-shadowing'])
 test('T21531', [ grep_errmsg(r'INLINE') ], compile, ['-ddump-ds'])
 test('T22521', normal, compile, [''])
 test('T23038', normal, compile_fail, [''])
+test('T22328', normal, compile, [''])