1 files changed, 19 insertions, 0 deletions
diff --git a/testsuite/tests/indexed-types/should_fail/T4254b.hs b/testsuite/tests/indexed-types/should_fail/T4254b.hs
index ffd117bc4c..44cdb5b602 100644
--- a/testsuite/tests/indexed-types/should_fail/T4254b.hs
+++ b/testsuite/tests/indexed-types/should_fail/T4254b.hs
@@ -11,3 +11,22 @@ fails :: forall a b. (a~Int,FD a b) => a -> Bool
 fails  = op
 -- Could fail: no proof that b~Bool
 -- But can also succeed; it's not a *wanted* constraint
+
+{- Interestingly, the ambiguity check for the type sig succeeds:
+
+[G] FD Int b
+[W] FD Int beta
+
+We get [W] beta~b; we unify immediately, and then solve.
+All before we interact the [W] FD Int beta with the
+top-level instances (which would give rise to [W] beta~Bool).
+
+One the other hand, from `fails = op` we get
+
+[G] FD Int b
+[W] FD Int Bool
+
+Interacting those two gives [W] b~Bool; bu this doesn't
+happen becase we now solve first.
+
+-}
+\ No newline at end of file