From 8fba47a7b415b4f299618418f1eda91e06aed93f Mon Sep 17 00:00:00 2001
From: robobun <117481402+robobun@users.noreply.github.com>
Date: Mon, 11 May 2026 14:10:28 +0000
Subject: [PATCH] Fix capacity miscalculation in ESMConditions.init when
 allow_addons is true

The expression `defaults.len + 2 + if (allow_addons) 1 else 0 + conditions.len`
was parsed as `defaults.len + 2 + (if (allow_addons) 1 else (0 + conditions.len))`,
dropping conditions.len from the reserved capacity when allow_addons was true
(the default). With enough user-supplied conditions, putAssumeCapacity would
overflow the map and crash.
---
 src/bundler/options.zig            |  6 +++---
 test/bundler/bun-build-api.test.ts | 12 ++++++++++++
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/src/bundler/options.zig b/src/bundler/options.zig
index daea5ced59a3..bddfece6147a 100644
--- a/src/bundler/options.zig
+++ b/src/bundler/options.zig
@@ -1087,9 +1087,9 @@ pub const ESMConditions = struct {
         var require_condition_map = ConditionsMap.init(allocator);
         var style_condition_map = ConditionsMap.init(allocator);
 
-        try default_condition_amp.ensureTotalCapacity(defaults.len + 2 + if (allow_addons) 1 else 0 + conditions.len);
-        try import_condition_map.ensureTotalCapacity(defaults.len + 2 + if (allow_addons) 1 else 0 + conditions.len);
-        try require_condition_map.ensureTotalCapacity(defaults.len + 2 + if (allow_addons) 1 else 0 + conditions.len);
+        try default_condition_amp.ensureTotalCapacity(defaults.len + 2 + @as(usize, @intFromBool(allow_addons)) + conditions.len);
+        try import_condition_map.ensureTotalCapacity(defaults.len + 2 + @as(usize, @intFromBool(allow_addons)) + conditions.len);
+        try require_condition_map.ensureTotalCapacity(defaults.len + 2 + @as(usize, @intFromBool(allow_addons)) + conditions.len);
         try style_condition_map.ensureTotalCapacity(defaults.len + 2 + conditions.len);
 
         import_condition_map.putAssumeCapacity("import", {});
diff --git a/test/bundler/bun-build-api.test.ts b/test/bundler/bun-build-api.test.ts
index c1112743f02c..f74ac1a86a48 100644
--- a/test/bundler/bun-build-api.test.ts
+++ b/test/bundler/bun-build-api.test.ts
@@ -70,6 +70,18 @@ describe("Bun.build", () => {
     throw new Error("should have thrown");
   });
 
+  test("many conditions does not crash", async () => {
+    const dir = tempDirWithFiles("bun-build-api-many-conditions", {
+      "index.js": `export default 1;`,
+    });
+    const build = await Bun.build({
+      entrypoints: [join(dir, "index.js")],
+      conditions: ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p"],
+    });
+    expect(build.success).toBe(true);
+    expect(build.outputs).toHaveLength(1);
+  });
+
   // https://github.com/oven-sh/bun/issues/12818
   test("sourcemap + build error crash case", async () => {
     const dir = tempDirWithFiles("build", {