Don't report instance constraints with fundeps as redundant

More subtlety due to functional dependencies.
Note [Redundant constraints in instance decls] in TcErrors.

Fixes Trac #10100.

2 files changed, 14 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T10100.hs b/testsuite/tests/typecheck/should_compile/T10100.hs
new file mode 100644
index 0000000000..b88803c633
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T10100.hs
@@ -0,0 +1,13 @@
+{-# LANGUAGE MultiParamTypeClasses  #-}
+{-# LANGUAGE FunctionalDependencies #-}
+{-# LANGUAGE FlexibleInstances      #-}
+{-# LANGUAGE UndecidableInstances   #-}
+
+module T10100 where
+
+data Zero
+data Succ a
+
+class Add a b ab | a b -> ab, a ab -> b
+instance Add Zero b b
+instance (Add a b ab) => Add (Succ a) b (Succ ab)
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index b792629b42..c1ed5790b4 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -443,3 +443,4 @@ test('T9971', normal, compile, [''])
 test('T9999', normal, compile, [''])
 test('T10031', normal, compile, [''])
 test('T10072', normal, compile_fail, [''])
+test('T10100', normal, compile, [''])