Add a test for #21278

testsuite/tests/linear/should_fail/T21278.hs

+{-# LANGUAGE LinearTypes #-}
+module T21278 where
+
+data C a = forall p. C (a %p -> a)
+
+f :: C a -> C a
+f b = C (\x -> case b of C g -> g x)

testsuite/tests/linear/should_fail/T21278.stderr

+
+T21278.hs:7:11: error: [GHC-25897]
+    • Couldn't match type ‘p’ with ‘Many’
+        arising from multiplicity of ‘x’
+      ‘p’ is a rigid type variable bound by
+        a pattern with constructor:
+          C :: forall a (p :: GHC.Types.Multiplicity). (a %p -> a) %1 -> C a,
+        in a case alternative
+        at T21278.hs:7:26-28
+    • In the first argument of ‘C’, namely
+        ‘(\ x -> case b of C g -> g x)’
+      In the expression: C (\ x -> case b of C g -> g x)
+      In an equation for ‘f’: f b = C (\ x -> case b of C g -> g x)

testsuite/tests/linear/should_fail/all.T

@@ -40,3 +40,4 @@ test('T18888_datakinds', normal, compile_fail, [''])
 test('T19120', normal, compile_fail, [''])
 test('T20083', normal, compile_fail, ['-XLinearTypes'])
 test('T19361', normal, compile_fail, [''])
+test('T21278', normal, compile_fail, ['-XLinearTypes'])