generic-witness-maybe-abi

Availability: opt-in ABI compatibility review, no policy required. Severity/confidence: low/high. Select with --detectors generic-witness-maybe-abi.

Under Compact 0.34.0/ZKIR2, concrete specializations of one generic witness dispatch to the same host method without a generic type argument. When its result is the canonical standard-library Maybe<T>, the generated wrapper checks value even when is_some is false. A bigint payload and a byte-array payload cannot use one fixed absent-result placeholder. This rule reports the static signature relationship, with the declaration, conflicting return types and source reference sites grouped into one finding per canonical witness template.

import CompactStandardLibrary;
witness lookup<T>(key: Bytes<4>): Maybe<T>;
export circuit getNumber(key: Bytes<4>): Maybe<Uint<64>> {
  return disclose(lookup<Uint<64>>(key));
}
export circuit getBytes(key: Bytes<4>): Maybe<Bytes<32>> {
  return disclose(lookup<Bytes<32>>(key));
}

A shared host handler may distinguish keys, private state, call order or other context and return the correct typed payload. This is not evidence that the handler fails. Review absent-result handling and the generated witness signatures. Use distinct host witness names when incompatible result representations should have independent handlers. Uint<64> and Field are a quiet lookalike because their ordinary payload guards both accept 0n.

The rule requires identical concrete parameter types and known disjoint bigint/byte-array result representations. Resolved import/type aliases retain identity; separate concrete module instances and different witness declarations are kept separate. It describes the entry-reachable source/schema graph, including constant-dead references and zero-iteration callbacks whose wrappers the qualified compiler retains. It does not assert a successful invocation or proof feasibility.

The compiler-only distinct-module probe also emits one shared host method spelling. This initial rule groups only one canonical template: a quiet distinct-instance control is a scope negative, not evidence of separate dispatch or ABI safety.