Report error on unused destructured parameters

Fixes #11795

Author: sheetalkamat

src/compiler/checker.ts

@@ -15832,12 +15832,12 @@ namespace ts {
                 for (const key in node.locals) {
                     const local = node.locals[key];
                     if (!local.isReferenced) {
-                        if (local.valueDeclaration && local.valueDeclaration.kind === SyntaxKind.Parameter) {
-                            const parameter = <ParameterDeclaration>local.valueDeclaration;
+                        if (local.valueDeclaration && getRootDeclaration(local.valueDeclaration).kind === SyntaxKind.Parameter) {
+                            const parameter = <ParameterDeclaration>getRootDeclaration(local.valueDeclaration);
                             if (compilerOptions.noUnusedParameters &&
                                 !isParameterPropertyDeclaration(parameter) &&
                                 !parameterIsThisKeyword(parameter) &&
-                                !parameterNameStartsWithUnderscore(parameter)) {
+                                !parameterNameStartsWithUnderscore(local.valueDeclaration.name)) {
                                 error(local.valueDeclaration.name, Diagnostics._0_is_declared_but_never_used, local.name);
                             }
                         }
@@ -15861,8 +15861,8 @@ namespace ts {
             error(node, Diagnostics._0_is_declared_but_never_used, name);
         }
 
-        function parameterNameStartsWithUnderscore(parameter: ParameterDeclaration) {
-            return parameter.name && isIdentifierThatStartsWithUnderScore(parameter.name);
+        function parameterNameStartsWithUnderscore(parameterName: DeclarationName) {
+            return parameterName && isIdentifierThatStartsWithUnderScore(parameterName);
         }
 
         function isIdentifierThatStartsWithUnderScore(node: Node) {

tests/baselines/reference/unusedDestructuringParameters.errors.txt

@@ -0,0 +1,15 @@
+tests/cases/compiler/unusedDestructuringParameters.ts(1,13): error TS6133: 'a' is declared but never used.
+tests/cases/compiler/unusedDestructuringParameters.ts(3,14): error TS6133: 'a' is declared but never used.
+
+
+==== tests/cases/compiler/unusedDestructuringParameters.ts (2 errors) ====
+    const f = ([a]) => { };
+                ~
+!!! error TS6133: 'a' is declared but never used.
+    f([1]);
+    const f2 = ({a}) => { };
+                 ~
+!!! error TS6133: 'a' is declared but never used.
+    f2({ a: 10 });
+    const f3 = ([_]) => { };
+    f3([10]);

tests/baselines/reference/unusedDestructuringParameters.js

@@ -2,7 +2,9 @@
 const f = ([a]) => { };
 f([1]);
 const f2 = ({a}) => { };
-f2({ a: 10 });
+f2({ a: 10 });
+const f3 = ([_]) => { };
+f3([10]);
 
 //// [unusedDestructuringParameters.js]
 var f = function (_a) {
@@ -13,3 +15,7 @@ var f2 = function (_a) {
     var a = _a.a;
 };
 f2({ a: 10 });
+var f3 = function (_a) {
+    var _ = _a[0];
+};
+f3([10]);

tests/baselines/reference/unusedDestructuringParameters.symbols

@@ -2,15 +2,11 @@ tests/cases/compiler/unusedParametersWithUnderscore.ts(2,12): error TS6133: 'a'
 tests/cases/compiler/unusedParametersWithUnderscore.ts(2,19): error TS6133: 'c' is declared but never used.
 tests/cases/compiler/unusedParametersWithUnderscore.ts(2,27): error TS6133: 'd' is declared but never used.
 tests/cases/compiler/unusedParametersWithUnderscore.ts(2,29): error TS6133: 'e___' is declared but never used.
-tests/cases/compiler/unusedParametersWithUnderscore.ts(6,14): error TS6133: '_a' is declared but never used.
-tests/cases/compiler/unusedParametersWithUnderscore.ts(6,18): error TS6133: '___b' is declared but never used.
-tests/cases/compiler/unusedParametersWithUnderscore.ts(9,14): error TS6133: '_a' is declared but never used.
-tests/cases/compiler/unusedParametersWithUnderscore.ts(9,19): error TS6133: '___b' is declared but never used.
 tests/cases/compiler/unusedParametersWithUnderscore.ts(12,16): error TS6133: 'arg' is declared but never used.
 tests/cases/compiler/unusedParametersWithUnderscore.ts(18,13): error TS6133: 'arg' is declared but never used.
 
 
-==== tests/cases/compiler/unusedParametersWithUnderscore.ts (10 errors) ====
+==== tests/cases/compiler/unusedParametersWithUnderscore.ts (6 errors) ====
 
     function f(a, _b, c, ___, d,e___, _f) {
                ~
@@ -25,17 +21,9 @@ tests/cases/compiler/unusedParametersWithUnderscore.ts(18,13): error TS6133: 'ar
 
 
     function f2({_a, __b}) {
-                 ~~
-!!! error TS6133: '_a' is declared but never used.
-                     ~~~
-!!! error TS6133: '___b' is declared but never used.
     }
 
     function f3([_a, ,__b]) {
-                 ~~
-!!! error TS6133: '_a' is declared but never used.
-                      ~~~
-!!! error TS6133: '___b' is declared but never used.
     }
 
     function f4(...arg) {

tests/baselines/reference/unusedDestructuringParameters.types

@@ -2,4 +2,6 @@
 const f = ([a]) => { };
 f([1]);
 const f2 = ({a}) => { };
-f2({ a: 10 });
+f2({ a: 10 });
+const f3 = ([_]) => { };
+f3([10]);