sx/issues/0178-protocol-impl-signature-mismatch-silent-miscompile.md

0178 — protocol impl method with a mismatched return/param TYPE silently miscompiles

RESOLVED. The issue-0176 conformance gate was name-only, so an impl P for T with a mismatched return/param type (or arity) built a wrong-ABI thunk that silently miscompiled (exit 0, wrong value). Fix (src/ir/lower/protocol.zig): firstUnimplementedMethod now validates each impl method's signature against the protocol declaration — arity (after self), every param type, and the return type — substituting protocol Self→concrete via resolveProtoTypeSubSelf (recurses through pointer/many-pointer/optional/slice/array so []Self↔[]T etc. match; conservative .unresolved for Self-in-generic-arg). Comparison is by structural formatTypeName (alias/module/spelling independent), and typesClearlyDiffer skips when either side has an unresolved leaf at any depth — biased against false-positives. Mismatch → located diagnostic. Verified by 3+3 adversarial reviews (a mid-fix []Self false-positive was found and closed); suite 792/0. Regressions: examples/diagnostics/1201-diagnostics-protocol-impl-signature-mismatch.sx (negative), examples/protocols/0420-protocol-self-in-slice-param.sx (positive). Known gaps (pre-existing, loud not silent — out of scope): a Self-through-generic-arg mismatch (Box(Self)) and by-value array protocol params ([2]Self) fail at LLVM verification, not silently.

Symptom

An impl P for T whose method has the right NAME but a mismatched return type or parameter type is accepted (it satisfies the issue-0176 conformance gate, which is name-based), and dispatch through the erased protocol silently produces the WRONG result (exit 0). No diagnostic. (Arity mismatch and #builtin-body mismatch fail loudly — exit 1 — and are not this bug; the TYPE-mismatch cases are silent.)

Reproduction

#import "modules/std.sx";
P :: protocol { val :: (self: *Self) -> i64; }
T :: struct { n: i64 = 7; }
impl P for T { val :: (self: *T) -> bool { return true; } }  // return type bool ≠ i64
main :: () {
  t := T.{ n = 7 };
  p : P = t;
  print("{}\n", p.val());   // prints "1" (the bool), silently wrong — no diagnostic
}

A parameter-type mismatch (x: bool where the protocol declares x: i64) similarly dispatches silently wrong.

Investigation prompt

The issue-0176 conformance gate (firstUnimplementedMethod in src/ir/lower/protocol.zig) checks method PRESENCE (and rejects type_params > 0), but does NOT check that the impl method's SIGNATURE (parameter types, arity, return type) matches the protocol method's declared signature. A mismatched-type impl builds a thunk that calls the impl with the wrong ABI, silently miscompiling. Add signature validation when registering / gating an impl method against its protocol method: compare the impl method's params (after the erased self) and return type against the protocol declaration, and emit a located diagnostic on mismatch (arity, param type, or return type). The protocol method declaration is in protocol_decl_map; the impl FnDecl is in fn_ast_map. Decide whether this lives in the conformance gate or in ProtocolResolver.registerImplBlock (src/ir/protocols.zig). Follow the no-silent-fallback rule. Verify: the repro is now a clean diagnostic (exit 1); a correctly-typed impl still works; add an examples/diagnostics/11xx-... negative regression. (Found during adversarial review of issue 0176.)