warn on missing unused imports in deshadowing phase, rather than throwing - fixes #928

src/ast/scopes/ModuleScope.js

@@ -22,8 +22,13 @@ export default class ModuleScope extends Scope {
 			specifier.module.getExports().forEach( name => {
 				names.set(name);
 			});
+
 			if ( specifier.name !== '*' ) {
 				const declaration = specifier.module.traceExport( specifier.name );
+				if ( !declaration ) {
+					this.module.bundle.onwarn( `Non-existent export '${specifier.name}' is imported from ${specifier.module.id} by ${this.module.id}` );
+					return;
+				}
 				const name = declaration.getName( true );
 				if ( name !== specifier.name ) {
 					names.set( declaration.getName( true ) );

test/function/warn-on-unused-missing-imports/_config.js

@@ -0,0 +1,12 @@
+const path = require( 'path' );
+const assert = require( 'assert' );
+
+module.exports = {
+	solo: true,
+	description: 'warns on missing (but unused) imports',
+	warnings: warnings => {
+		assert.deepEqual( warnings, [
+			`Non-existent export 'b' is imported from ${path.resolve(__dirname, 'foo.js')} by ${path.resolve(__dirname, 'main.js')}`
+		]);
+	}
+};

test/function/warn-on-unused-missing-imports/foo.js

@@ -0,0 +1 @@
+export const a = 42;

test/function/warn-on-unused-missing-imports/main.js

@@ -0,0 +1,3 @@
+import { a, b } from './foo.js';
+
+assert.equal( a, 42 );