Comments about -Wredundant-constraints

compiler/typecheck/TcErrors.hs

@@ -367,6 +367,7 @@ reportImplic ctxt implic@(Implic { ic_skols = tvs, ic_given = given
                     _                             -> []
 
 warnRedundantConstraints :: ReportErrCtxt -> TcLclEnv -> SkolemInfo -> [EvVar] -> TcM ()
+-- See Note [Tracking redundant constraints] in TcSimplify
 warnRedundantConstraints ctxt env info ev_vars
  | null redundant_evs
  = return ()

compiler/typecheck/TcSimplify.hs

@@ -1511,13 +1511,23 @@ works:
 
 ----- Shortcomings
 
-Consider (see Trac #9939)
-    f2 :: (Eq a, Ord a) => a -> a -> Bool
+After I introduced -Wredundant-constraints there was extensive discussion
+about cases where it reported a redundant constraint but the programmer
+really wanted it.  See
+
+  * #11370 (removed it from -Wdefault)
+  * #10635 (removed it from -Wall as well)
+  * #12142
+  * #11474, #10100 (class not used, but its fundeps are)
+  * #11099 (redundant, but still desired)
+  * #10183 (constraint necessary to exclude omitted case)
+
+  * #9939:     f2 :: (Eq a, Ord a) => a -> a -> Bool
                -- Ord a redundant, but Eq a is reported
               f2 x y = (x == y)
 
-We report (Eq a) as redundant, whereas actually (Ord a) is.  But it's
-really not easy to detect that!
+    We report (Eq a) as redundant, whereas actually (Ord a) is.
+    But it's really not easy to detect that!
 
 
 Note [Cutting off simpl_loop]