1 files changed, 35 insertions, 0 deletions
diff --git a/testsuite/tests/typecheck/should_fail/T18851.hs b/testsuite/tests/typecheck/should_fail/T18851.hs
new file mode 100644
index 0000000000..bba609a5ce
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+++ b/testsuite/tests/typecheck/should_fail/T18851.hs
@@ -0,0 +1,35 @@
+{-# LANGUAGE FunctionalDependencies, FlexibleInstances, UndecidableInstances,
+             ScopedTypeVariables, TypeFamilies, TypeApplications,
+             FlexibleContexts, AllowAmbiguousTypes, ExtendedDefaultRules #-}
+
+module T18851 where
+
+default (Int)
+
+type family C_FD a
+class C_FD a ~ b => C a b
+
+type instance C_FD Int = Bool -- just for Show (C_FD Int)
+instance C Int b => C Int b
+
+class IsInt int
+instance int ~ Int => IsInt int
+
+data A
+instance Show A where
+  show _ = "A"
+data B
+instance Show B where
+  show _ = "B"
+
+f :: forall a b c int
+  .  ( Show c, Num int
+     , C int a, C int b, C int c
+     -- , c ~ C_FD int -- add this to get rid of ambiguity error
+     )
+  => String
+f = show (undefined :: c)
+
+-- blows up at run time once type checks
+g :: String
+g = f @A @B