From 532b35675d5567b1e22c2616444c18eb41434c07 Mon Sep 17 00:00:00 2001
From: alexander-akait <sheo13666q@gmail.com>
Date: Tue, 7 Oct 2025 17:11:22 +0300
Subject: [PATCH] fix: defer import mangling

---
 lib/RuntimeTemplate.js                         | 11 ++++++++++-
 .../defer-import/import-attributes/file.ext    |  4 ++++
 .../defer-import/import-attributes/index.js    | 18 ++++++++++++++++++
 .../import-attributes/test.filter.js           |  5 +++++
 .../import-attributes/webpack.config.js        |  9 +++++++++
 5 files changed, 46 insertions(+), 1 deletion(-)
 create mode 100644 test/configCases/defer-import/import-attributes/file.ext
 create mode 100644 test/configCases/defer-import/import-attributes/index.js
 create mode 100644 test/configCases/defer-import/import-attributes/test.filter.js
 create mode 100644 test/configCases/defer-import/import-attributes/webpack.config.js

diff --git a/lib/RuntimeTemplate.js b/lib/RuntimeTemplate.js
index 5aa4f2e254e..6c4b0aba981 100644
--- a/lib/RuntimeTemplate.js
+++ b/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;
 					}
diff --git a/test/configCases/defer-import/import-attributes/file.ext b/test/configCases/defer-import/import-attributes/file.ext
new file mode 100644
index 00000000000..879aeeaf4bf
--- /dev/null
+++ b/test/configCases/defer-import/import-attributes/file.ext
@@ -0,0 +1,4 @@
+{
+  "foo": "bar",
+  "nested": { "foo": "bar" }
+}
diff --git a/test/configCases/defer-import/import-attributes/index.js b/test/configCases/defer-import/import-attributes/index.js
new file mode 100644
index 00000000000..60df4a38608
--- /dev/null
+++ b/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");
+});
diff --git a/test/configCases/defer-import/import-attributes/test.filter.js b/test/configCases/defer-import/import-attributes/test.filter.js
new file mode 100644
index 00000000000..03c5581fcb2
--- /dev/null
+++ b/test/configCases/defer-import/import-attributes/test.filter.js
@@ -0,0 +1,5 @@
+"use strict";
+
+const supportsTextDecoder = require("../../../helpers/supportsTextDecoder");
+
+module.exports = () => supportsTextDecoder();
diff --git a/test/configCases/defer-import/import-attributes/webpack.config.js b/test/configCases/defer-import/import-attributes/webpack.config.js
new file mode 100644
index 00000000000..c3897eb5054
--- /dev/null
+++ b/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
+	}
+};