Fix unreachable code detection persisting after incremental edits

src/compiler/binder.ts

@@ -1091,6 +1091,11 @@ function createBinder(): (file: SourceFile, options: CompilerOptions) => void {
         // and set it before we descend into nodes that could actually be part of an assignment pattern.
         inAssignmentPattern = false;
 
+        // Clear Unreachable flag from previous binding (for incremental scenarios)
+        if (isPotentiallyExecutableNode(node)) {
+            (node as Mutable<Node>).flags &= ~NodeFlags.Unreachable;
+        }
+
         if (currentFlow === unreachableFlow) {
             if (canHaveFlowNode(node)) {
                 node.flowNode = undefined;

tests/cases/fourslash/unreachableCodeAfterEdit.ts

@@ -0,0 +1,44 @@
+/// <reference path="fourslash.ts" />
+
+// @allowUnreachableCode: false
+// @lib: es2015
+
+// @Filename: /base/browser/browser.ts
+//// export const isStandalone = true;
+
+// @Filename: /base/browser/dom.ts
+//// export function addDisposableListener() {}
+
+// @Filename: /base/browser/window.ts
+//// export const mainWindow = {} as Window;
+
+// @Filename: /workbench.ts
+//// /*before*/import { isStandalone } from './base/browser/browser';
+//// import { addDisposableListener } from './base/browser/dom';
+//// import { mainWindow } from './base/browser/window';
+//// 
+//// interface ISecretStorageCrypto {
+////     seal(data: string): Promise<string>;
+////     unseal(data: string): Promise<string>;
+//// }
+//// 
+//// export class TransparentCrypto implements ISecretStorageCrypto {
+////     async seal(data: string): Promise<string> {
+////         return data;
+////     }
+////     async unseal(data: string): Promise<string> {
+////         return data;
+////     }
+//// }
+
+verify.numberOfErrorsInCurrentFile(0);
+
+goTo.marker("before");
+edit.insert("throw new Error('foo');\n");
+
+verify.numberOfErrorsInCurrentFile(1);
+
+goTo.marker("before");
+edit.deleteAtCaret("throw new Error('foo');\n".length);
+
+verify.numberOfErrorsInCurrentFile(0);

tests/cases/fourslash/unusedLabelAfterEdit.ts

@@ -0,0 +1,23 @@
+/// <reference path="fourslash.ts" />
+
+// @allowUnusedLabels: false
+
+//// myLabel: while (true) {
+////     if (Math.random() > 0.5) {
+////         /*marker*/break myLabel;
+////     }
+//// }
+
+verify.numberOfErrorsInCurrentFile(0);
+
+goTo.marker("marker");
+edit.deleteAtCaret("break myLabel;".length);
+edit.insert("break;");
+
+verify.numberOfErrorsInCurrentFile(1);
+
+goTo.marker("marker");
+edit.deleteAtCaret("break;".length);
+edit.insert("break myLabel;");
+
+verify.numberOfErrorsInCurrentFile(0);