fix(install): exclude optional peers resolved against dev deps in --prod

When running `bun install --production`, optional peer dependencies of
production packages (e.g. typescript as an optional peer of @prisma/client)
were incorrectly installed if they also appeared in the root devDependencies.

During the `.resolvable` tree-building phase, optional peers get resolved
against any matching dependency in the tree, including dev deps. In the
subsequent `.filter` phase, the root dev dependency is correctly filtered
out, but the optional peer retains its resolved package ID and passes
through the filter since remote_package_features.peer_dependencies is true.

The fix adds a check in the filter phase: when an optional peer dependency
from a non-root package matches a root dev dependency and dev_dependencies
are disabled (--prod mode), it is filtered out.

Closes #26837

Co-Authored-By: Claude <noreply@anthropic.com>

src/install/lockfile/Tree.zig

@@ -394,6 +394,21 @@ pub fn isFilteredDependencyOrWorkspace(
     // Filtering only applies to the root package dependencies. Also
     // --filter has a different meaning if a new package is being installed.
     if (manager.subcommand != .install or parent_pkg_id != 0) {
+        // Optional peer dependencies can get resolved against root dev
+        // dependencies during the `.resolvable` phase. In `--prod` mode
+        // (dev_dependencies disabled), these optional peers should not
+        // pull in packages that would otherwise only exist as dev deps.
+        if (parent_pkg_id != 0 and dep.behavior.isOptionalPeer() and
+            !manager.options.local_package_features.dev_dependencies)
+        {
+            const root_dep_list = pkgs.items(.dependencies)[0];
+            const root_deps = lockfile.buffers.dependencies.items[root_dep_list.begin()..root_dep_list.end()];
+            for (root_deps) |root_dep| {
+                if (root_dep.name_hash == dep.name_hash and root_dep.behavior.isDev()) {
+                    return true;
+                }
+            }
+        }
         return false;
     }
 

test/regression/issue/26837.test.ts

@@ -0,0 +1,45 @@
+// https://github.com/oven-sh/bun/issues/26837
+// Test that `bun install --production` does not install optional peer
+// dependencies that are also listed as devDependencies. When a production
+// dependency (e.g. @prisma/client) has an optional peer (e.g. typescript),
+// and that same package appears in the root devDependencies, `--production`
+// should NOT install it.
+
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+
+test("--production should not install optional peers that match devDependencies", async () => {
+  using dir = tempDir("issue-26837", {
+    "package.json": JSON.stringify({
+      name: "test-issue-26837",
+      dependencies: {
+        "@prisma/client": "^6.3.1",
+      },
+      devDependencies: {
+        typescript: "^5.7.3",
+      },
+    }),
+  });
+
+  // Run bun install --production
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "install", "--production"],
+    cwd: String(dir),
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  // typescript should NOT be installed since it's a devDependency
+  // and we're installing with --production
+  const output = stdout + stderr;
+  expect(output).not.toContain("error:");
+
+  // Check that typescript is not in node_modules
+  const typescriptExists = await Bun.file(`${dir}/node_modules/typescript/package.json`).exists();
+  expect(typescriptExists).toBe(false);
+
+  expect(exitCode).toBe(0);
+});