Add test case for #8743

which only occurs when the instance being compiled is also present from
a .hs-boot file.

testsuite/tests/stranal/should_compile/T8743.hs

+module T8743 where
+
+-- Without the following import, it does not fail
+import {-# SOURCE #-} T8743 ()
+
+-- [()] required, () does not work.
+class ToRow a where toRow :: a -> [()]
+
+instance ToRow (Maybe a) where
+    toRow Nothing  = [()]
+    toRow (Just _) = [()]

testsuite/tests/stranal/should_compile/T8743.hs-boot

+module T8743 where
+class ToRow a
+instance ToRow (Maybe a)

testsuite/tests/stranal/should_compile/all.T

@@ -18,3 +18,5 @@ test('newtype', req_profiling, compile, ['-prof -auto-all'])
 test('T1988', normal, compile, [''])
 test('T8467', normal, compile, [''])
 test('T8037', normal, compile, [''])
+test('T8743', [ expect_broken(8743), extra_clean(['T8743.o-boot', 'T8743.hi-boot']) ], multimod_compile, ['T8743', '-v0'])
+