fix: defer import mangling (#19988)

lib/RuntimeTemplate.js

@@ -949,7 +949,16 @@ class RuntimeTemplate {
 			// when the defaultInterop is used (when a ESM imports a CJS module),
 			if (exportName.length > 0 && exportName[0] === "default") {
 				if (isDeferred && exportsType !== "namespace") {
-					const access = `${importVar}.a${propertyAccess(exportName, 1)}`;
+					const exportsInfo = moduleGraph.getExportsInfo(module);
+					const name = exportName.slice(1);
+					const used = exportsInfo.getUsedName(name, runtime);
+					if (!used) {
+						const comment = Template.toNormalComment(
+							`unused export ${propertyAccess(exportName)}`
+						);
+						return `${comment} undefined`;
+					}
+					const access = `${importVar}.a${propertyAccess(used)}`;
 					if (isCall || asiSafe === undefined) {
 						return access;
 					}

test/configCases/defer-import/import-attributes/file.ext

@@ -0,0 +1,4 @@
+{
+  "foo": "bar",
+  "nested": { "foo": "bar" }
+}

test/configCases/defer-import/import-attributes/index.js

@@ -0,0 +1,18 @@
+import defer * as mod1 from "./file.ext" with { type: "bytes" };
+import defer * as mod2 from "./file.ext" with { type: "json" };
+import * as mod3 from "./file.ext" with { type: "bytes" };
+import * as mod4 from "./file.ext" with { type: "json" };
+
+it("should work with defer and import attributes", () => {
+	const decoder = new TextDecoder('utf-8');
+	const mod1Decoded = JSON.parse(decoder.decode(mod1.default));
+	expect(mod1Decoded.foo).toBe("bar");
+	expect(mod1Decoded.nested.foo).toBe("bar");
+	expect(mod2.default.foo).toBe("bar");
+	expect(mod2.default.nested.foo).toBe("bar");
+	const mod2Decoded = JSON.parse(decoder.decode(mod3.default));
+	expect(mod2Decoded.foo).toBe("bar");
+	expect(mod2Decoded.nested.foo).toBe("bar");
+	expect(mod4.default.foo).toBe("bar");
+	expect(mod4.default.nested.foo).toBe("bar");
+});

test/configCases/defer-import/import-attributes/test.filter.js

@@ -0,0 +1,5 @@
+"use strict";
+
+const supportsTextDecoder = require("../../../helpers/supportsTextDecoder");
+
+module.exports = () => supportsTextDecoder();

test/configCases/defer-import/import-attributes/webpack.config.js

@@ -0,0 +1,9 @@
+"use strict";
+
+/** @type {import("../../../../").Configuration} */
+module.exports = {
+	target: [`async-node${process.versions.node.split(".").map(Number)[0]}`],
+	experiments: {
+		deferImport: true
+	}
+};