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Turbopack: ProductionBrowserSourceMaps Disables Dead-Import Elimination For Compiler.Define-Folded Typeof Guards

Enabling productionBrowserSourceMaps in Turbopack prevents tree-shaking of side-effect-free imports referenced only in typeof-guarded dead branches after compiler.define replacement, causing unreachable modules to be bundled.

highConfidence 85%Next.js Affected V16.3.0 Affected V16.3.2 Affected V16.4.0-Canary.3

Origin Analysis

Turbopack's source map generation treats original import bindings as live references for mapping generated code, preventing dead-code elimination of modules whose only use is inside typeof __FLAG__ === 'undefined' || __FLAG__ guards folded by compiler.define. The import is kept in the module graph even though the call site is eliminated, resulting in the module subtree being emitted.

1. Create a Next.js app with a local package fake-sdk with sideEffects: false, containing import { feature } from './feature.js' and a guard if (typeof __MY_FLAG__ === 'undefined' || __MY_FLAG__) { integrations.push(feature()); }. 2. Set productionBrowserSourceMaps: true and compiler: { define: { __MY_FLAG__: false } } in next.config.mjs. 3. Run next build. 4. Grep for FEATURE_MARKER_SHOULD_BE_TREESHAKEN in .next/static/chunks/; it is present when source maps are enabled, absent when disabled.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

Disabling Turbopack may reduce build performance and remove access to Turbopack-specific optimizations. It may also be unsupported in future Next.js versions if webpack fallback is removed. Use only as a temporary hotfix; revert to Turbopack once the underlying tree-shaking bug is fixed upstream. Test build output and source maps after applying.