SC2321: Warn about redundant $(()) in arr[$((i))]=x (ref: koalaman#1666)

CHANGELOG.md

@@ -4,6 +4,7 @@
 - SC2317: Warn about unreachable commands
 - SC2318: Warn about backreferences in 'declare x=1 y=$x'
 - SC2319/SC2320: Warn when $? refers to echo/printf/[ ]/[[ ]]/test
+- SC2321: Suggest removing $((..)) in array[$((idx))]=val
 
 ### Fixed
 - SC2086: Now uses DFA to make more accurate predictions about values

src/ShellCheck/Analytics.hs

@@ -206,6 +206,7 @@ nodeChecks = [
     ,checkCommandIsUnreachable
     ,checkSpacefulnessCfg
     ,checkOverwrittenExitCode
+    ,checkUnnecessaryArithmeticExpansionIndex
     ]
 
 optionalChecks = map fst optionalTreeChecks
@@ -4913,5 +4914,22 @@ checkOverwrittenExitCode params t =
             _ -> False
 
 
+prop_checkUnnecessaryArithmeticExpansionIndex1 = verify checkUnnecessaryArithmeticExpansionIndex "a[$((1+1))]=n"
+prop_checkUnnecessaryArithmeticExpansionIndex2 = verifyNot checkUnnecessaryArithmeticExpansionIndex "a[1+1]=n"
+prop_checkUnnecessaryArithmeticExpansionIndex3 = verifyNot checkUnnecessaryArithmeticExpansionIndex "a[$(echo $((1+1)))]=n"
+checkUnnecessaryArithmeticExpansionIndex params t =
+    case t of
+        T_Assignment _ mode var [TA_Sequence _ [ TA_Expansion _ [expansion@(T_DollarArithmetic id _)]]] val ->
+            styleWithFix id 2321 "Array indices are already arithmetic contexts. Prefer removing the $(( and ))." $ fix id
+        _ -> return ()
+
+  where
+    fix id =
+        fixWith [
+            replaceStart id params 3 "", -- Remove "$(("
+            replaceEnd id params 2 ""    -- Remove "))"
+        ]
+
+
 return []
 runTests =  $( [| $(forAllProperties) (quickCheckWithResult (stdArgs { maxSuccess = 1 }) ) |])