From 350117624262cf12857b0d43896bfc06ade47570 Mon Sep 17 00:00:00 2001
From: Claude Bot <claude-bot@bun.sh>
Date: Thu, 22 Jan 2026 13:20:01 +0000
Subject: [PATCH] fix(macros): prevent incorrect const inlining of
 macro-returned objects/arrays

Objects and arrays returned from macros were being treated as "const values"
for constant propagation, causing each reference to be replaced with a fresh
copy. This broke mutation semantics where obj.foo = x followed by reading
obj.foo would read from a different object.

Fixes #26362

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
---
 src/ast/Expr.zig                    |  7 ++-
 test/regression/issue/26362.test.ts | 86 +++++++++++++++++++++++++++++
 2 files changed, 91 insertions(+), 2 deletions(-)
 create mode 100644 test/regression/issue/26362.test.ts

diff --git a/src/ast/Expr.zig b/src/ast/Expr.zig
index aa9f7eb5b5..fb8336d85e 100644
--- a/src/ast/Expr.zig
+++ b/src/ast/Expr.zig
@@ -2657,8 +2657,11 @@ pub const Data = union(Tag) {
             .e_inlined_enum,
             => true,
             .e_string => |str| str.next == null,
-            .e_array => |array| array.was_originally_macro,
-            .e_object => |object| object.was_originally_macro,
+            // Objects and arrays are NOT const values because they are mutable.
+            // Inlining them would create fresh copies at each reference site,
+            // breaking mutation semantics (e.g., obj.foo = 1 would modify a
+            // different object than a subsequent read of obj.foo).
+            // See: https://github.com/oven-sh/bun/issues/26362
             else => false,
         };
     }
diff --git a/test/regression/issue/26362.test.ts b/test/regression/issue/26362.test.ts
new file mode 100644
index 0000000000..05d27f55ca
--- /dev/null
+++ b/test/regression/issue/26362.test.ts
@@ -0,0 +1,86 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+
+test("objects returned from macros should be mutable", async () => {
+  using dir = tempDir("issue-26362", {
+    "macro.ts": `export const getObj = () => ({});`,
+    "index.ts": `
+import { getObj } from "./macro.ts" with { type: "macro" };
+
+const obj = getObj();
+
+// Object should be extensible (not frozen/sealed)
+if (!Object.isExtensible(obj)) {
+  throw new Error("Object is not extensible");
+}
+
+// Should be able to add properties
+obj.foo = "bar";
+
+// Should be able to read back the property we just set
+if (obj.foo !== "bar") {
+  throw new Error("Property was not set correctly, got: " + obj.foo);
+}
+
+console.log("success");
+`,
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "index.ts"],
+    cwd: String(dir),
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stderr).toBe("");
+  expect(stdout).toInclude("success");
+  expect(exitCode).toBe(0);
+});
+
+test("arrays returned from macros should be mutable", async () => {
+  using dir = tempDir("issue-26362-array", {
+    "macro.ts": `export const getArr = () => [];`,
+    "index.ts": `
+import { getArr } from "./macro.ts" with { type: "macro" };
+
+const arr = getArr();
+
+// Array should be extensible
+if (!Object.isExtensible(arr)) {
+  throw new Error("Array is not extensible");
+}
+
+// Should be able to push elements
+arr.push("first");
+
+// Should be able to read back what we pushed
+if (arr[0] !== "first") {
+  throw new Error("Element was not pushed correctly, got: " + arr[0]);
+}
+
+if (arr.length !== 1) {
+  throw new Error("Array length is wrong, got: " + arr.length);
+}
+
+console.log("success");
+`,
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "index.ts"],
+    cwd: String(dir),
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stderr).toBe("");
+  expect(stdout).toInclude("success");
+  expect(exitCode).toBe(0);
+});