From 859785aa6b2f299031d6a07c7d1ecbc2921a2098 Mon Sep 17 00:00:00 2001
From: robobun <117481402+robobun@users.noreply.github.com>
Date: Thu, 14 May 2026 04:18:19 +0000
Subject: [PATCH] Fix capacity underflow in ESMConditions.init when conditions
 are passed

The expression 'if (allow_addons) 1 else 0 + conditions.len' was parsed
as 'if (allow_addons) 1 else (0 + conditions.len)', so when allow_addons
was true (the default), conditions.len was never counted toward the map
capacity. Passing more than a couple of custom conditions to Bun.build
would trip the putAssumeCapacity assert in debug builds and write past
the reserved capacity in release builds.
---
 src/bundler/options.zig            |  6 +++---
 test/bundler/bun-build-api.test.ts | 11 +++++++++++
 2 files changed, 14 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..6c7e3b2525dd 100644
--- a/test/bundler/bun-build-api.test.ts
+++ b/test/bundler/bun-build-api.test.ts
@@ -422,6 +422,17 @@ describe("Bun.build", () => {
     expect(x.logs[0].position).toEqual(null);
   });
 
+  test.concurrent("many custom conditions do not crash", async () => {
+    const dir = tempDirWithFiles("bun-build-many-conditions", {
+      "index.ts": `export const a = 1;`,
+    });
+    const x = await Bun.build({
+      entrypoints: [join(dir, "index.ts")],
+      conditions: ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p"],
+    });
+    expect(x.success).toBe(true);
+  });
+
   test.concurrent("warnings do not fail a build", async () => {
     const x = await Bun.build({
       entrypoints: [join(import.meta.dir, "./fixtures/jsx-warning/index.jsx")],