Foundry Update Fixes Enum Mutation Bugs in Smart Contract Fuzzing

This is not cosmetic. Enums drive state-transition logic across upgradeable proxies, role-based access, and dispatch tables. The mutation strategy determines whether the fuzzer reaches adversarial edge cases or silently skips them. Bound violations turn the invariant under test into fiction.

Mechanics of the fix

The release trail — nightly builds 08e262a80 through 6df18e8ae — traces a rolling chain of corpus-logic updates. The headline change: the mutator no longer generates out-of-bound enum values. Bound enforcement moves from the contract layer to the fuzzer layer.

Without the patch, fuzzers rely on randomized input mutation to explore state space. An out-of-bound enum emission produces one of two failure modes: silent revert in the test harness, or a vacuous assertion pass. Both corrupt coverage metrics. Both mask real invariants. For enum-dispatched branches — common in proxy upgrades, role-gated execution, and state-machine contracts — the old behavior meant either under-tested paths or tests running against impossible states. The patch constrains the mutation space to valid enum members, restoring the determinism that invariant testing demands.

What to verify

Pull the latest release. Check forge --version. Re-run your invariant suites. Expected outcome: a reduced revert rate on enum-typed inputs, tighter coverage of valid branches, and a cleaner mutation corpus.

For contracts that gate execution on enum dispatch — upgradeable proxies, RBAC patterns, state machines — the old behavior let adversarial paths slip through untested. The new behavior forces the fuzzer to stay within declared bounds, producing a more honest state-transition graph.

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Pre-flight checklist:

• Confirm the patched build via forge --version.
• Re-run invariant suites with bounded enum mutation enabled.
• Audit custom fuzzer harnesses for manual enum construction that bypasses the new bound logic.
• Compare coverage reports before and after the upgrade.
• Post-upgrade enum-bound violations are test-harness bugs, not contract vulnerabilities.