Merge pull request #11790 from Microsoft/allowUnderScoreForIn

Allow unused locals in for in or for of that start with _

src/compiler/checker.ts

@@ -15836,15 +15836,31 @@ namespace ts {
                             }
                         }
                         else if (compilerOptions.noUnusedLocals) {
-                            forEach(local.declarations, d => error(d.name || d, Diagnostics._0_is_declared_but_never_used, local.name));
+                            forEach(local.declarations, d => errorUnusedLocal(d.name || d, local.name));
                         }
                     }
                 }
             }
         }
 
+        function errorUnusedLocal(node: Node, name: string) {
+            if (isIdentifierThatStartsWithUnderScore(node)) {
+                const declaration = getRootDeclaration(node.parent);
+                if (declaration.kind === SyntaxKind.VariableDeclaration &&
+                    (declaration.parent.parent.kind === SyntaxKind.ForInStatement ||
+                    declaration.parent.parent.kind === SyntaxKind.ForOfStatement)) {
+                    return;
+                }
+            }
+            error(node, Diagnostics._0_is_declared_but_never_used, name);
+        }
+
         function parameterNameStartsWithUnderscore(parameter: ParameterDeclaration) {
-            return parameter.name && parameter.name.kind === SyntaxKind.Identifier && (<Identifier>parameter.name).text.charCodeAt(0) === CharacterCodes._;
+            return parameter.name && isIdentifierThatStartsWithUnderScore(parameter.name);
+        }
+
+        function isIdentifierThatStartsWithUnderScore(node: Node) {
+            return node.kind === SyntaxKind.Identifier && (<Identifier>node).text.charCodeAt(0) === CharacterCodes._;
         }
 
         function checkUnusedClassMembers(node: ClassDeclaration | ClassExpression): void {

tests/baselines/reference/unusedLocalsStartingWithUnderscore.errors.txt

@@ -0,0 +1,18 @@
+tests/cases/compiler/unusedLocalsStartingWithUnderscore.ts(7,9): error TS6133: '_' is declared but never used.
+
+
+==== tests/cases/compiler/unusedLocalsStartingWithUnderscore.ts (1 errors) ====
+    
+    for (const _ of []) { }
+    
+    for (const _ in []) { }
+    
+    namespace M {
+        let _;
+            ~
+!!! error TS6133: '_' is declared but never used.
+        for (const _ of []) { }
+    
+        for (const _ in []) { }
+    }
+        

tests/baselines/reference/unusedLocalsStartingWithUnderscore.js

@@ -0,0 +1,27 @@
+//// [unusedLocalsStartingWithUnderscore.ts]
+
+for (const _ of []) { }
+
+for (const _ in []) { }
+
+namespace M {
+    let _;
+    for (const _ of []) { }
+
+    for (const _ in []) { }
+}
+    
+
+//// [unusedLocalsStartingWithUnderscore.js]
+for (var _i = 0, _a = []; _i < _a.length; _i++) {
+    var _ = _a[_i];
+}
+for (var _ in []) { }
+var M;
+(function (M) {
+    var _;
+    for (var _i = 0, _a = []; _i < _a.length; _i++) {
+        var _1 = _a[_i];
+    }
+    for (var _2 in []) { }
+})(M || (M = {}));

tests/cases/compiler/unusedLocalsStartingWithUnderscore.ts

@@ -0,0 +1,13 @@
+//@noUnusedLocals:true
+
+for (const _ of []) { }
+
+for (const _ in []) { }
+
+namespace M {
+    let _;
+    for (const _ of []) { }
+
+    for (const _ in []) { }
+}
+