Add testcases for already fixed #16432

They were fixed by 40c7daed.

Fixes #16432

testsuite/tests/quantified-constraints/T23333.hs

 {-# LANGUAGE QuantifiedConstraints #-}
+{-# LANGUAGE GADTs, DataKinds #-}
 module T23333 where
 
+import Data.Kind
+import Data.Coerce
+
 foo1 :: (forall y. Bool ~ y) => z -> Bool
 foo1 x = not x
 
 foo2 :: (forall y. y ~ Bool) => z -> Bool
 foo2 x = not x
+
+-- Testcases from #16432
+t1 :: forall f b. (forall a. Coercible (f a) a) => b -> f b
+t1 = coerce
+
+data U :: () -> Type where
+ MkU :: Int -> U '()
+
+t2 :: forall n res. (('()~n) => (Int~res)) => U n -> res
+t2 (MkU n) = n
+
+t3 :: ((Bool~Bool) => (Char~res)) => res
+t3 = 'a'