Add testcase for #23192

This issue around solving of constraints arising from superclass
expansion using other constraints also borned from superclass expansion
was the topic of commit aed1974e. That commit made sure we don't emit
a "redundant constraint" warning in a situation in which removing the
constraint would cause errors.

Fixes #23192

testsuite/tests/typecheck/should_compile/T23192.hs

+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# OPTIONS_GHC -Werror=redundant-constraints #-}
+
+module EventThing where
+
+class Monad m => Event m where
+  thingsForEvent :: m [Int]
+
+class Monad m => Thingy m where
+  thingies :: m [Int]
+
+-- Check that we don't get a redundant constraint warning for "Monad m".
+-- See #19690.
+instance (Monad m, Event m) => Thingy m where
+  thingies = thingsForEvent

testsuite/tests/typecheck/should_compile/all.T

@@ -870,3 +870,4 @@ test('T21443', normal, compile, [''])
 test('QualifiedRecordUpdate',
     [ extra_files(['QualifiedRecordUpdate_aux.hs']) ]
     , multimod_compile, ['QualifiedRecordUpdate', '-v0'])
+test('T23192', normal, compile, [''])