Narrow types by typeof operands with extra parenthesis (microsoft#60928)

Author: Andarist

src/compiler/binder.ts

@@ -1318,9 +1318,11 @@ function createBinder(): (file: SourceFile, options: CompilerOptions) => void {
             case SyntaxKind.ExclamationEqualsToken:
             case SyntaxKind.EqualsEqualsEqualsToken:
             case SyntaxKind.ExclamationEqualsEqualsToken:
-                return isNarrowableOperand(expr.left) || isNarrowableOperand(expr.right) ||
-                    isNarrowingTypeofOperands(expr.right, expr.left) || isNarrowingTypeofOperands(expr.left, expr.right) ||
-                    (isBooleanLiteral(expr.right) && isNarrowingExpression(expr.left) || isBooleanLiteral(expr.left) && isNarrowingExpression(expr.right));
+                const left = skipParentheses(expr.left);
+                const right = skipParentheses(expr.right);
+                return isNarrowableOperand(left) || isNarrowableOperand(right) ||
+                    isNarrowingTypeofOperands(right, left) || isNarrowingTypeofOperands(left, right) ||
+                    (isBooleanLiteral(right) && isNarrowingExpression(left) || isBooleanLiteral(left) && isNarrowingExpression(right));
             case SyntaxKind.InstanceOfKeyword:
                 return isNarrowableOperand(expr.left);
             case SyntaxKind.InKeyword:

tests/baselines/reference/narrowingTypeofParenthesized1.symbols

@@ -0,0 +1,30 @@
+//// [tests/cases/compiler/narrowingTypeofParenthesized1.ts] ////
+
+=== narrowingTypeofParenthesized1.ts ===
+// https://github.com/microsoft/TypeScript/issues/42203
+
+declare const foo: string;
+>foo : Symbol(foo, Decl(narrowingTypeofParenthesized1.ts, 2, 13))
+
+if ((typeof foo) === "string") {
+>foo : Symbol(foo, Decl(narrowingTypeofParenthesized1.ts, 2, 13))
+
+  foo;
+>foo : Symbol(foo, Decl(narrowingTypeofParenthesized1.ts, 2, 13))
+
+} else {
+  foo;
+>foo : Symbol(foo, Decl(narrowingTypeofParenthesized1.ts, 2, 13))
+}
+
+if (typeof foo === ("string")) {
+>foo : Symbol(foo, Decl(narrowingTypeofParenthesized1.ts, 2, 13))
+
+  foo;
+>foo : Symbol(foo, Decl(narrowingTypeofParenthesized1.ts, 2, 13))
+
+} else {
+  foo;
+>foo : Symbol(foo, Decl(narrowingTypeofParenthesized1.ts, 2, 13))
+}
+

tests/baselines/reference/narrowingTypeofParenthesized1.types

@@ -0,0 +1,53 @@
+//// [tests/cases/compiler/narrowingTypeofParenthesized1.ts] ////
+
+=== narrowingTypeofParenthesized1.ts ===
+// https://github.com/microsoft/TypeScript/issues/42203
+
+declare const foo: string;
+>foo : string
+>    : ^^^^^^
+
+if ((typeof foo) === "string") {
+>(typeof foo) === "string" : boolean
+>                          : ^^^^^^^
+>(typeof foo) : "string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>             : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>typeof foo : "string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>           : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>foo : string
+>    : ^^^^^^
+>"string" : "string"
+>         : ^^^^^^^^
+
+  foo;
+>foo : string
+>    : ^^^^^^
+
+} else {
+  foo;
+>foo : never
+>    : ^^^^^
+}
+
+if (typeof foo === ("string")) {
+>typeof foo === ("string") : boolean
+>                          : ^^^^^^^
+>typeof foo : "string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>           : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>foo : string
+>    : ^^^^^^
+>("string") : "string"
+>           : ^^^^^^^^
+>"string" : "string"
+>         : ^^^^^^^^
+
+  foo;
+>foo : string
+>    : ^^^^^^
+
+} else {
+  foo;
+>foo : never
+>    : ^^^^^
+}
+

tests/cases/compiler/narrowingTypeofParenthesized1.ts

@@ -0,0 +1,18 @@
+// @strict: true
+// @noEmit: true
+
+// https://github.com/microsoft/TypeScript/issues/42203
+
+declare const foo: string;
+
+if ((typeof foo) === "string") {
+  foo;
+} else {
+  foo;
+}
+
+if (typeof foo === ("string")) {
+  foo;
+} else {
+  foo;
+}