fixes #48630 array binding pattern with only OmittedExpressions does not check RHS of for-of loop (#49008)

* fix RHS of for..of loop not evaluated when LHS is array binding element with OmittedExpression

* expand widened type check

* add more test cases

* update code with suggestions

* Make test target es2015

Co-authored-by: Andrew Branch <andrew@wheream.io>

src/compiler/checker.ts

@@ -37933,7 +37933,7 @@ namespace ts {
             // For a binding pattern, validate the initializer and exit
             if (isBindingPattern(node.name)) {
                 const needCheckInitializer = node.initializer && node.parent.parent.kind !== SyntaxKind.ForInStatement;
-                const needCheckWidenedType = node.name.elements.length === 0;
+                const needCheckWidenedType = !some(node.name.elements, not(isOmittedExpression));
                 if (needCheckInitializer || needCheckWidenedType) {
                     // Don't validate for-in initializer as it is already an error
                     const widenedType = getWidenedTypeForVariableLikeDeclaration(node);

test.js

@@ -0,0 +1,9 @@
+"use strict";
+exports.__esModule = true;
+exports.main = void 0;
+function main() {
+    for (var _i = 0, doesNotExist_1 = doesNotExist; _i < doesNotExist_1.length; _i++) {
+        var _a = doesNotExist_1[_i];
+    }
+}
+exports.main = main;

test.ts

@@ -0,0 +1,4 @@
+export function main() {
+    for (const [,] of doesNotExist) {
+    }
+}

tests/baselines/reference/omittedExpressionForOfLoop.errors.txt

@@ -0,0 +1,26 @@
+tests/cases/compiler/omittedExpressionForOfLoop.ts(1,19): error TS2304: Cannot find name 'doesNotExist'.
+tests/cases/compiler/omittedExpressionForOfLoop.ts(4,19): error TS2532: Object is possibly 'undefined'.
+tests/cases/compiler/omittedExpressionForOfLoop.ts(7,12): error TS2488: Type 'never' must have a '[Symbol.iterator]()' method that returns an iterator.
+tests/cases/compiler/omittedExpressionForOfLoop.ts(10,12): error TS2488: Type 'never' must have a '[Symbol.iterator]()' method that returns an iterator.
+
+
+==== tests/cases/compiler/omittedExpressionForOfLoop.ts (4 errors) ====
+    for (const [,] of doesNotExist) {
+                      ~~~~~~~~~~~~
+!!! error TS2304: Cannot find name 'doesNotExist'.
+    }
+    
+    for (const [,] of undefined) {
+                      ~~~~~~~~~
+!!! error TS2532: Object is possibly 'undefined'.
+    }
+    
+    for (const [,] of []) {
+               ~~~
+!!! error TS2488: Type 'never' must have a '[Symbol.iterator]()' method that returns an iterator.
+    }
+    
+    for (const [] of []) {
+               ~~
+!!! error TS2488: Type 'never' must have a '[Symbol.iterator]()' method that returns an iterator.
+    }

tests/baselines/reference/omittedExpressionForOfLoop.js

@@ -0,0 +1,23 @@
+//// [omittedExpressionForOfLoop.ts]
+for (const [,] of doesNotExist) {
+}
+
+for (const [,] of undefined) {
+}
+
+for (const [,] of []) {
+}
+
+for (const [] of []) {
+}
+
+//// [omittedExpressionForOfLoop.js]
+"use strict";
+for (const [,] of doesNotExist) {
+}
+for (const [,] of undefined) {
+}
+for (const [,] of []) {
+}
+for (const [] of []) {
+}

tests/baselines/reference/omittedExpressionForOfLoop.symbols

@@ -0,0 +1,13 @@
+=== tests/cases/compiler/omittedExpressionForOfLoop.ts ===
+for (const [,] of doesNotExist) {
+}
+
+for (const [,] of undefined) {
+>undefined : Symbol(undefined)
+}
+
+for (const [,] of []) {
+}
+
+for (const [] of []) {
+}

tests/baselines/reference/omittedExpressionForOfLoop.types

@@ -0,0 +1,19 @@
+=== tests/cases/compiler/omittedExpressionForOfLoop.ts ===
+for (const [,] of doesNotExist) {
+> : undefined
+>doesNotExist : any
+}
+
+for (const [,] of undefined) {
+> : undefined
+>undefined : undefined
+}
+
+for (const [,] of []) {
+> : undefined
+>[] : never[]
+}
+
+for (const [] of []) {
+>[] : never[]
+}

tests/cases/compiler/omittedExpressionForOfLoop.ts

@@ -0,0 +1,14 @@
+// @strict: true
+// @target: es2015
+
+for (const [,] of doesNotExist) {
+}
+
+for (const [,] of undefined) {
+}
+
+for (const [,] of []) {
+}
+
+for (const [] of []) {
+}