prevent {export foo as default} from creating a live binding (#860)

src/Module.js

@@ -170,7 +170,13 @@ export default class Module {
 						throw new Error( `A module cannot have multiple exports with the same name ('${exportedName}')` );
 					}
 
-					this.exports[ exportedName ] = { localName };
+					// `export { default as foo }` – special case. We want importers
+					// to use the UnboundDefaultExport proxy, not the original declaration
+					if ( exportedName === 'default' ) {
+						this.exports[ exportedName ] = { localName: 'default' };
+					} else {
+						this.exports[ exportedName ] = { localName };
+					}
 				});
 			} else {
 				this.bundle.onwarn( `Module ${this.id} has an empty export declaration` );

src/ast/nodes/ExportNamedDeclaration.js

@@ -1,17 +1,59 @@
+import { find } from '../../utils/array.js';
 import Node from '../Node.js';
 
+class UnboundDefaultExport {
+	constructor ( original ) {
+		this.original = original;
+		this.name = original.name;
+	}
+
+	activate () {
+		if ( this.activated ) return;
+		this.activated = true;
+
+		this.original.activate();
+	}
+
+	addReference ( reference ) {
+		this.name = reference.name;
+		this.original.addReference( reference );
+	}
+
+	bind ( scope ) {
+		this.original.bind( scope );
+	}
+
+	gatherPossibleValues ( values ) {
+		this.original.gatherPossibleValues( values );
+	}
+
+	getName ( es ) {
+		if ( this.original && !this.original.isReassigned ) {
+			return this.original.getName( es );
+		}
+
+		return this.name;
+	}
+}
+
 export default class ExportNamedDeclaration extends Node {
 	initialise ( scope ) {
+		this.scope = scope;
 		this.isExportDeclaration = true;
-		if ( this.declaration ) {
-			this.declaration.initialise( scope );
+
+		// special case – `export { foo as default }` should not create a live binding
+		const defaultExport = find( this.specifiers, specifier => specifier.exported.name === 'default' );
+		if ( defaultExport ) {
+			const declaration = this.scope.findDeclaration( defaultExport.local.name );
+			this.defaultExport = new UnboundDefaultExport( declaration );
+			scope.declarations.default = this.defaultExport;
 		}
+
+		if ( this.declaration ) this.declaration.initialise( scope );
 	}
 
 	bind ( scope ) {
-		if ( this.declaration ) {
-			this.declaration.bind( scope );
-		}
+		if ( this.declaration ) this.declaration.bind( scope );
 	}
 
 	render ( code, es ) {
@@ -19,7 +61,20 @@ export default class ExportNamedDeclaration extends Node {
 			code.remove( this.start, this.declaration.start );
 			this.declaration.render( code, es );
 		} else {
-			code.remove( this.leadingCommentStart || this.start, this.next || this.end );
+			const start = this.leadingCommentStart || this.start;
+			const end = this.next || this.end;
+
+			if ( this.defaultExport ) {
+				const name = this.defaultExport.getName( es );
+				const originalName = this.defaultExport.original.getName( es );
+
+				if ( name !== originalName ) {
+					code.overwrite( start, end, `var ${name} = ${originalName};` );
+					return;
+				}
+			}
+
+			code.remove( start, end );
 		}
 	}
 }

test/function/export-as-default/_config.js

@@ -1,6 +1,5 @@
 var assert = require( 'assert' );
 
 module.exports = {
-	solo: true,
 	description: 'export { foo as default } does not create a live binding'
 };