Avoid adding redundant brackets around lambdas (#1646)

Author: zliu41

src/GHC/Util/HsExpr.hs

@@ -243,7 +243,7 @@ niceLambdaR parent = go
           -- e.g., parent = `f \x -> g 3 x`; e = `g 3`.
           -- Brackets should be placed around `e` to produce `f (g 3)` instead of `f g 3`.
           addBrackets = case parent of
-            Just p -> isApp p && not (isVar e)
+            Just p -> isApp p && not (isVar e) && not (isLambda e)
             Nothing -> False
           e' = if addBrackets then mkHsPar e else e
           tpl = if addBrackets then "(a)" else "a"

src/Hint/Lambda.hs

@@ -43,6 +43,7 @@ f = g \x -> h 3 x -- (h 3)
 f = g (\x -> h 3 x) -- h 3
 f = g \x -> (`h` 3) x -- (`h` 3)
 f = g \x -> h x -- h
+f = ($ 42) \_ -> pure ()
 f = foo (flip op x) -- (`op` x)
 f = foo (flip op x) -- @Message Use section
 f = foo (flip x y) -- (`x` y)