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diff --git a/testsuite/tests/indexed-types/should_compile/T4120.hs b/testsuite/tests/indexed-types/should_compile/T4120.hs
new file mode 100644
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+++ b/testsuite/tests/indexed-types/should_compile/T4120.hs
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+{-# LANGUAGE Rank2Types, TypeFamilies #-}
+
+-- Unification yielding a coercion under a forall
+
+module Data.Vector.Unboxed where
+
+import Control.Monad.ST ( ST )
+
+
+data MVector s a = MV
+data Vector a    = V
+
+type family Mutable (v :: * -> *) :: * -> * -> *
+type instance Mutable Vector = MVector
+
+create :: (forall s. MVector s a) -> Int
+create = create1
+-- Here we get  Couldn't match expected type `forall s. MVector s a'
+--                          with actual type `forall s. Mutable Vector s a1'
+-- Reason: when unifying under a for-all we don't solve type 
+--         equalities.  Think more about this.
+
+create1 :: (forall s. Mutable Vector s a) -> Int
+create1 = error "urk"
+
+