Do not entirely move else branch if another else branch might accidentally be referenced.

src/ast/nodes/IfStatement.ts

@@ -5,6 +5,7 @@ import { DeoptimizableEntity } from '../DeoptimizableEntity';
 import { BROKEN_FLOW_NONE, HasEffectsContext, InclusionContext } from '../ExecutionContext';
 import { EMPTY_PATH, SHARED_RECURSION_TRACKER } from '../utils/PathTracker';
 import { LiteralValueOrUnknown, UnknownValue } from '../values';
+import BlockStatement from './BlockStatement';
 import * as NodeType from './NodeType';
 import { ExpressionNode, IncludeChildren, StatementBase, StatementNode } from './shared/Node';
 
@@ -86,6 +87,8 @@ export default class IfStatement extends StatementBase implements DeoptimizableE
 						code.prependLeft(this.alternate.start, ' ');
 					}
 					this.alternate.render(code, options);
+				} else if (this.shouldKeepAlternateBranch()) {
+					code.overwrite(this.alternate.start, this.alternate.end, ';');
 				} else {
 					code.remove(this.consequent.end, this.alternate.end);
 				}
@@ -142,4 +145,18 @@ export default class IfStatement extends StatementBase implements DeoptimizableE
 				context.brokenFlow < consequentBrokenFlow ? context.brokenFlow : consequentBrokenFlow;
 		}
 	}
+
+	private shouldKeepAlternateBranch() {
+		let currentParent = this.parent;
+		do {
+			if (currentParent instanceof IfStatement && currentParent.alternate) {
+				return true;
+			}
+			if (currentParent instanceof BlockStatement) {
+				return false;
+			}
+			currentParent = (currentParent as any).parent;
+		} while (currentParent);
+		return false;
+	}
 }

test/function/samples/simplified-nested-if/_config.js

@@ -0,0 +1,7 @@
+module.exports = {
+	description:
+		'do not catch else branches from parent if statements when simplifiying if-statements',
+	context: {
+		unknown: true
+	}
+};

test/function/samples/simplified-nested-if/main.js

@@ -0,0 +1,24 @@
+export let foo = 0;
+
+if (unknown)
+	for (var x = 1; x-- > 0; )
+		if (!unknown) foo++;
+		else 'Side effect free code to be dropped';
+else throw new Error("Shouldn't be reached");
+
+if (unknown)
+	for (var x = 1; x-- > 0; )
+		if (foo++, false) foo++;
+		else 'Side effect free code to be dropped';
+else throw new Error("Shouldn't be reached");
+
+if (unknown) {
+	for (var x = 1; x-- > 0; )
+		if (!unknown) foo++;
+		else 'Side effect free code to be dropped';
+} else throw new Error("Shouldn't be reached");
+
+if (unknown)
+	for (var x = 1; x-- > 0; )
+		if (!unknown) foo++;
+		else 'Side effect free code to be dropped';