Add a test for #7848

testsuite/tests/parser/should_fail/T7848.hs

+
+module T7848 where
+
+data A = (:&&) Int Int | A Int Int
+
+x (+) ((&)@z) ((:&&) a b) (c :&& d) (e `A` f) (A g h) = y
+  where infixl 3 `y`
+        y _ = (&)
+        {-# INLINE (&) #-}
+        {-# SPECIALIZE (&) :: a #-}
+        (&) = x

testsuite/tests/parser/should_fail/T7848.stderr

+
+T7848.hs:10:9:
+    Couldn't match expected type ‛forall a. a’
+                with actual type ‛t -> t1 -> A -> A -> A -> A -> t2’
+    Relevant bindings include
+      x :: t -> t1 -> A -> A -> A -> A -> t2 (bound at T7848.hs:6:1)
+      (+) :: t (bound at T7848.hs:6:3)
+      (&) :: t1 (bound at T7848.hs:6:8)
+      z :: t1 (bound at T7848.hs:6:12)
+    In the SPECIALISE pragma {-# SPECIALIZE (&) :: a #-}
+    In an equation for ‛x’:
+        x (+) ((&)@z) ((:&&) a b) (c :&& d) (e `A` f) (A g h)
+          = y
+          where
+              infixl 3 `y`
+              y _ = (&)
+              {-# INLINE (&) #-}
+              {-# SPECIALIZE (&) :: a #-}
+              (&) = x

testsuite/tests/parser/should_fail/all.T

@@ -77,3 +77,4 @@ test('ParserNoMultiWayIf', when(compiler_lt('ghc', '7.5'), skip), compile_fail,
 
 test('T5425', normal, compile_fail, [''])
 test('T984', normal, compile_fail, [''])
+test('T7848', normal, compile_fail, ['-dppr-user-length=100'])