Improve redundant-constraints warning

Previously, we reported things wrong with

  f :: (Eq a, Ord a) => a -> Bool
  f x = x == x

saying that Eq a was redundant. This is fixed now, along with
some simplification in Note [Replacement vs keeping]. There's
a tiny bit of extra complexity in setImplicationStatus, but
it's explained in Note [Tracking redundant constraints].

Close #20602

4 files changed, 72 insertions, 2 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T20602.hs b/testsuite/tests/typecheck/should_compile/T20602.hs
new file mode 100644
index 0000000000..03dd5af458
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T20602.hs
@@ -0,0 +1,29 @@
+{-# LANGUAGE TypeFamilies #-}
+
+{-# OPTIONS_GHC -Wredundant-constraints #-}
+
+module T20602 where
+
+f :: (Eq a, Ord a) => a -> Bool
+f x = x == x
+
+g :: (Eq a, Ord a) => a -> Bool
+g x = x > x
+
+h :: (Eq a, Ord a) => a -> Bool
+h x = x == x && x > x
+
+j :: (Eq a, a ~ b, Eq b) => a -> Bool
+j x = x == x
+
+k :: (Eq a, a ~ b, Eq b) => a -> Bool
+k _ = True
+
+l :: (Eq a, Ord a) => a -> Bool
+l _ = True
+
+m :: (a ~ b) => a -> a
+m x = x
+
+n :: (Eq a, b ~ a) => a -> Bool
+n x = (==) x x
diff --git a/testsuite/tests/typecheck/should_compile/T20602.stderr b/testsuite/tests/typecheck/should_compile/T20602.stderr
new file mode 100644
index 0000000000..5dacc5be5a
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T20602.stderr
@@ -0,0 +1,40 @@
+
+T20602.hs:7:6: warning: [-Wredundant-constraints]
+    • Redundant constraint: Ord a
+    • In the type signature for:
+           f :: forall a. (Eq a, Ord a) => a -> Bool
+
+T20602.hs:10:6: warning: [-Wredundant-constraints]
+    • Redundant constraint: Eq a
+    • In the type signature for:
+           g :: forall a. (Eq a, Ord a) => a -> Bool
+
+T20602.hs:13:6: warning: [-Wredundant-constraints]
+    • Redundant constraint: Eq a
+    • In the type signature for:
+           h :: forall a. (Eq a, Ord a) => a -> Bool
+
+T20602.hs:16:6: warning: [-Wredundant-constraints]
+    • Redundant constraint: Eq b
+    • In the type signature for:
+           j :: forall a b. (Eq a, a ~ b, Eq b) => a -> Bool
+
+T20602.hs:19:6: warning: [-Wredundant-constraints]
+    • Redundant constraints: (Eq a, a ~ b, Eq b)
+    • In the type signature for:
+           k :: forall a b. (Eq a, a ~ b, Eq b) => a -> Bool
+
+T20602.hs:22:6: warning: [-Wredundant-constraints]
+    • Redundant constraints: (Eq a, Ord a)
+    • In the type signature for:
+           l :: forall a. (Eq a, Ord a) => a -> Bool
+
+T20602.hs:25:6: warning: [-Wredundant-constraints]
+    • Redundant constraint: a ~ b
+    • In the type signature for:
+           m :: forall a b. (a ~ b) => a -> a
+
+T20602.hs:28:6: warning: [-Wredundant-constraints]
+    • Redundant constraint: b ~ a
+    • In the type signature for:
+           n :: forall a b. (Eq a, b ~ a) => a -> Bool
diff --git a/testsuite/tests/typecheck/should_compile/T9939.stderr b/testsuite/tests/typecheck/should_compile/T9939.stderr
index 3d4c964a15..3067cddbf6 100644
--- a/testsuite/tests/typecheck/should_compile/T9939.stderr
+++ b/testsuite/tests/typecheck/should_compile/T9939.stderr
@@ -5,7 +5,7 @@ T9939.hs:6:7: warning: [-Wredundant-constraints]
            f1 :: forall a. (Eq a, Ord a) => a -> a -> Bool
 
 T9939.hs:10:7: warning: [-Wredundant-constraints]
-    • Redundant constraint: Eq a
+    • Redundant constraint: Ord a
     • In the type signature for:
            f2 :: forall a. (Eq a, Ord a) => a -> a -> Bool
 
@@ -15,6 +15,6 @@ T9939.hs:14:7: warning: [-Wredundant-constraints]
            f3 :: forall a b. (Eq a, a ~ b, Eq b) => a -> b -> Bool
 
 T9939.hs:21:7: warning: [-Wredundant-constraints]
-    • Redundant constraint: Eq a
+    • Redundant constraint: Eq b
     • In the type signature for:
            f4 :: forall a b. (Eq a, Eq b) => a -> b -> Equal a b -> Bool
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index 7061f26779..9fbf11b641 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -572,6 +572,7 @@ test('T13458', normal, compile, [''])
 test('T13490', normal, compile, [''])
 test('T13474', normal, compile, [''])
 test('T13524', normal, compile, [''])
+test('T20602', normal, compile, [''])
 test('T13509', normal, compile, [''])
 test('T13526', normal, compile, [''])
 test('T13594', normal, compile_fail, [''])