Add regression test for #19921

testsuite/tests/quantified-constraints/T19921.hs

+{-# LANGUAGE
+    QuantifiedConstraints
+  , StandaloneKindSignatures
+  , TypeOperators
+  , GADTs
+  , ConstraintKinds
+  , RankNTypes
+  , UndecidableInstances
+  , ImpredicativeTypes
+#-}
+module Typelevel.Constraint.Repro where
+
+import Data.Kind (Constraint, Type)
+
+type Dict :: Constraint -> Type
+data Dict c
+  where
+  Dict :: c => Dict c
+
+type (⇒) :: Constraint -> Constraint -> Constraint
+type c ⇒ d = c => d
+infixr 0 ⇒
+
+type (\/) :: Constraint -> Constraint -> Constraint
+type a \/ b = (forall r. (a ⇒ r, b ⇒ r) ⇒ r)
+infixr 5 \/
+
+dict :: Dict ((x \/ y) \/ z ⇒ x \/ y \/ z)
+dict = Dict

testsuite/tests/quantified-constraints/T19921.stderr

+
+T19921.hs:29:8: error:
+    • Could not deduce: r arising from a use of ‘Dict’
+      from the context: (x \/ y) \/ z
+        bound by a quantified context at T19921.hs:1:1
+      or from: (x ⇒ r, (y \/ z) ⇒ r)
+        bound by a quantified context at T19921.hs:1:1
+    • In the expression: Dict
+      In an equation for ‘dict’: dict = Dict

testsuite/tests/quantified-constraints/all.T

@@ -30,3 +30,4 @@ test('T17267d', normal, compile_and_run, [''])
 test('T17267e', normal, compile_fail, [''])
 test('T17458', normal, compile_fail, [''])
 test('T18432', normal, compile, [''])
+test('T19921', normal, compile_fail, [''])