Add a test for Trac #10806

testsuite/tests/indexed-types/should_compile/T10806.hs

+{-# LANGUAGE GADTs, ExplicitNamespaces, TypeOperators, DataKinds  #-}
+
+module T10806 where
+
+import GHC.TypeLits (Nat, type (<=))
+
+data Q a where
+    Q :: (a <= b, b <= c) => proxy a -> proxy b -> Q c
+
+triggersLoop :: Q b -> Q b -> Bool
+triggersLoop (Q _ _) (Q _ _) = print 'x' 'y'

testsuite/tests/indexed-types/should_compile/T10806.stderr

+
+T10806.hs:11:32: error:
+    Couldn't match expected type ‘Char -> Bool’
+                with actual type ‘IO ()’
+    The function ‘print’ is applied to two arguments,
+    but its type ‘Char -> IO ()’ has only one
+    In the expression: print 'x' 'y'
+    In an equation for ‘triggersLoop’:
+        triggersLoop (Q _ _) (Q _ _) = print 'x' 'y'

testsuite/tests/indexed-types/should_compile/all.T

@@ -262,4 +262,4 @@ test('T10507', normal, compile, [''])
 test('T10634', normal, compile, [''])
 test('T10713', normal, compile, [''])
 test('T10753', normal, compile, [''])
-
+test('T10806', normal, compile_fail, [''])