Add regression test for #8142

testsuite/tests/typecheck/should_fail/T8142.hs

+{-# LANGUAGE TypeFamilies #-}
+
+module T8142 where
+
+tracer :: (Functor f, Coinductive f) => (c -> f c) -> (c -> f c)
+tracer = h where h = (\(_, b) -> ((outI . fmap h) b)) . out
+
+class Functor g => Coinductive g where
+  type Nu g :: *
+  out     :: Nu g -> g (Nu g)
+  outI    :: g (Nu g) -> Nu g

testsuite/tests/typecheck/should_fail/T8142.stderr

+
+T8142.hs:6:18:
+    Couldn't match type ‛Nu f0’ with ‛Nu f’
+    NB: ‛Nu’ is a type function, and may not be injective
+    The type variable ‛f0’ is ambiguous
+    Expected type: a -> Nu f
+      Actual type: a -> Nu f0
+    When checking that ‛h’
+      has the inferred type ‛forall (f :: * -> *) a. a -> Nu f’
+    Probable cause: the inferred type is ambiguous
+    In an equation for ‛tracer’:
+        tracer
+          = h
+          where
+              h = (\ (_, b) -> ((outI . fmap h) b)) . out
+
+T8142.hs:6:57:
+    Occurs check: cannot construct the infinite type: a ~ f1 a
+    Expected type: a -> (t0, f1 a)
+      Actual type: Nu ((,) t0) -> (t0, Nu ((,) t0))
+    Relevant bindings include h :: a -> Nu f1 (bound at T8142.hs:6:18)
+    In the second argument of ‛(.)’, namely ‛out’
+    In the expression: (\ (_, b) -> ((outI . fmap h) b)) . out

testsuite/tests/typecheck/should_fail/all.T

- 
+
 test('tcfail001', normal, compile_fail, [''])
 test('tcfail002', normal, compile_fail, [''])
 test('tcfail003', normal, compile_fail, [''])
@@ -217,7 +217,7 @@ test('T3950', normal, compile_fail, [''])
 test('T3966', normal, compile_fail, ['-O'])
 test('IPFail', normal, compile_fail, [''])
 
-test('T3468', extra_clean(['T3468.hi-boot', 'T3468.o-boot']), 
+test('T3468', extra_clean(['T3468.hi-boot', 'T3468.o-boot']),
      multimod_compile_fail, ['T3468', '-v0'])
 test('T2846b', normal, compile_fail, [''])
 test('FrozenErrorTests', normal, compile_fail, [''])
@@ -311,3 +311,4 @@ test('T7869', normal, compile_fail, [''])
 test('T7892', normal, compile_fail, [''])
 test('T7809', normal, compile_fail, [''])
 test('T7989', normal, compile_fail, [''])
+test('T8142', normal, compile_fail, [''])