Don't add diagnostic on unused import starting with underscore (#24958)

* Don't add diagnostic on unused import starting with underscore

* Fix lint

src/compiler/checker.ts

@@ -22895,7 +22895,11 @@ namespace ts {
                 }
 
                 for (const declaration of local.declarations) {
-                    if (isAmbientModule(declaration)) continue;
+                    if (isAmbientModule(declaration) ||
+                        (isVariableDeclaration(declaration) && isForInOrOfStatement(declaration.parent.parent) || isImportedDeclaration(declaration)) && isIdentifierThatStartsWithUnderScore(declaration.name!)) {
+                        continue;
+                    }
+
                     if (isImportedDeclaration(declaration)) {
                         addToGroup(unusedImports, importClauseFromImported(declaration), declaration, getNodeId);
                     }
@@ -22907,9 +22911,7 @@ namespace ts {
                         }
                     }
                     else if (isVariableDeclaration(declaration)) {
-                        if (!isIdentifierThatStartsWithUnderScore(declaration.name) || !isForInOrOfStatement(declaration.parent.parent)) {
-                            addToGroup(unusedVariables, declaration.parent, declaration, getNodeId);
-                        }
+                        addToGroup(unusedVariables, declaration.parent, declaration, getNodeId);
                     }
                     else {
                         const parameter = local.valueDeclaration && tryGetRootParameterDeclaration(local.valueDeclaration);

tests/baselines/reference/unusedLocalsStartingWithUnderscore.errors.txt

@@ -1,12 +1,17 @@
-tests/cases/compiler/unusedLocalsStartingWithUnderscore.ts(6,9): error TS6133: '_' is declared but its value is never read.
+/a.ts(7,11): error TS6133: '_ns' is declared but its value is never read.
+/a.ts(8,9): error TS6133: '_' is declared but its value is never read.
 
 
-==== tests/cases/compiler/unusedLocalsStartingWithUnderscore.ts (1 errors) ====
+==== /a.ts (2 errors) ====
+    import * as _ from "./a";
+    
     for (const _ of []) { }
     
     for (const _ in []) { }
     
-    namespace M {
+    namespace _ns {
+              ~~~
+!!! error TS6133: '_ns' is declared but its value is never read.
         let _;
             ~
 !!! error TS6133: '_' is declared but its value is never read.
@@ -14,4 +19,4 @@ tests/cases/compiler/unusedLocalsStartingWithUnderscore.ts(6,9): error TS6133: '
     
         for (const _ in []) { }
     }
-        
+    

tests/baselines/reference/unusedLocalsStartingWithUnderscore.js

@@ -1,26 +1,30 @@
-//// [unusedLocalsStartingWithUnderscore.ts]
+//// [a.ts]
+import * as _ from "./a";
+
 for (const _ of []) { }
 
 for (const _ in []) { }
 
-namespace M {
+namespace _ns {
     let _;
     for (const _ of []) { }
 
     for (const _ in []) { }
 }
-    
 
-//// [unusedLocalsStartingWithUnderscore.js]
+
+//// [a.js]
+"use strict";
+exports.__esModule = true;
 for (var _i = 0, _a = []; _i < _a.length; _i++) {
-    var _ = _a[_i];
+    var _1 = _a[_i];
 }
-for (var _ in []) { }
-var M;
-(function (M) {
+for (var _2 in []) { }
+var _ns;
+(function (_ns) {
     var _;
     for (var _i = 0, _a = []; _i < _a.length; _i++) {
-        var _1 = _a[_i];
+        var _3 = _a[_i];
     }
-    for (var _2 in []) { }
-})(M || (M = {}));
+    for (var _4 in []) { }
+})(_ns || (_ns = {}));

tests/baselines/reference/unusedLocalsStartingWithUnderscore.symbols

@@ -1,20 +1,23 @@
-=== tests/cases/compiler/unusedLocalsStartingWithUnderscore.ts ===
+=== /a.ts ===
+import * as _ from "./a";
+>_ : Symbol(_, Decl(a.ts, 0, 6))
+
 for (const _ of []) { }
->_ : Symbol(_, Decl(unusedLocalsStartingWithUnderscore.ts, 0, 10))
+>_ : Symbol(_, Decl(a.ts, 2, 10))
 
 for (const _ in []) { }
->_ : Symbol(_, Decl(unusedLocalsStartingWithUnderscore.ts, 2, 10))
+>_ : Symbol(_, Decl(a.ts, 4, 10))
 
-namespace M {
->M : Symbol(M, Decl(unusedLocalsStartingWithUnderscore.ts, 2, 23))
+namespace _ns {
+>_ns : Symbol(_ns, Decl(a.ts, 4, 23))
 
     let _;
->_ : Symbol(_, Decl(unusedLocalsStartingWithUnderscore.ts, 5, 7))
+>_ : Symbol(_, Decl(a.ts, 7, 7))
 
     for (const _ of []) { }
->_ : Symbol(_, Decl(unusedLocalsStartingWithUnderscore.ts, 6, 14))
+>_ : Symbol(_, Decl(a.ts, 8, 14))
 
     for (const _ in []) { }
->_ : Symbol(_, Decl(unusedLocalsStartingWithUnderscore.ts, 8, 14))
+>_ : Symbol(_, Decl(a.ts, 10, 14))
 }
-    
+

tests/baselines/reference/unusedLocalsStartingWithUnderscore.types

@@ -1,4 +1,7 @@
-=== tests/cases/compiler/unusedLocalsStartingWithUnderscore.ts ===
+=== /a.ts ===
+import * as _ from "./a";
+>_ : typeof _
+
 for (const _ of []) { }
 >_ : any
 >[] : undefined[]
@@ -7,8 +10,8 @@ for (const _ in []) { }
 >_ : string
 >[] : undefined[]
 
-namespace M {
->M : typeof M
+namespace _ns {
+>_ns : typeof _ns
 
     let _;
 >_ : any
@@ -21,4 +24,4 @@ namespace M {
 >_ : string
 >[] : undefined[]
 }
-    
+

tests/cases/compiler/unusedLocalsStartingWithUnderscore.ts

@@ -1,13 +1,15 @@
-//@noUnusedLocals:true
+// @noUnusedLocals:true
+
+// @Filename: /a.ts
+import * as _ from "./a";
 
 for (const _ of []) { }
 
 for (const _ in []) { }
 
-namespace M {
+namespace _ns {
     let _;
     for (const _ of []) { }
 
     for (const _ in []) { }
 }
-