3c738695dc
Erasing a type to a protocol when it conforms only via a free function (not an explicit impl P for T) built a vtable of unreachable thunks -> SIGABRT on first dispatch, with no diagnostic. Per specs.md erasure is impl-driven, not structural, so the erasure was never valid. Add a conformance gate (firstUnimplementedMethod in buildProtocolValue, src/ir/lower/protocol.zig): emit a located diagnostic when a protocol method has no reachable impl, or when an impl method introduces its own type params (signature mismatch — it bails lazyLowerFunction and would reach the unreachable thunk). A std.debug.panic tripwire guards the diagnostics==null path so a non-conforming erasure can never silently ship as undef. Gate<->thunk equivalence verified bidirectional. Regressions: protocols/0419 (positive struct-field dispatch), diagnostics/1197 (no-impl) + 1198 (generic-method signature mismatch). Updated memory/0808 (it erased a non-conforming type that never dispatched). Verified by 3+1 adversarial reviews, suite 788/0. Filed adjacent bug 0178 (protocol impl method type-mismatch silent miscompile).
46 lines
2.1 KiB
Markdown
46 lines
2.1 KiB
Markdown
# 0178 — protocol impl method with a mismatched return/param TYPE silently miscompiles
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## Symptom
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An `impl P for T` whose method has the right NAME but a mismatched return type or
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parameter type is accepted (it satisfies the issue-0176 conformance gate, which
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is name-based), and dispatch through the erased protocol silently produces the
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WRONG result (exit 0). No diagnostic. (Arity mismatch and `#builtin`-body
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mismatch fail loudly — exit 1 — and are not this bug; the TYPE-mismatch cases are
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silent.)
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## Reproduction
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```sx
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#import "modules/std.sx";
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P :: protocol { val :: (self: *Self) -> i64; }
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T :: struct { n: i64 = 7; }
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impl P for T { val :: (self: *T) -> bool { return true; } } // return type bool ≠ i64
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main :: () {
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t := T.{ n = 7 };
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p : P = t;
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print("{}\n", p.val()); // prints "1" (the bool), silently wrong — no diagnostic
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}
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```
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A parameter-type mismatch (`x: bool` where the protocol declares `x: i64`)
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similarly dispatches silently wrong.
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## Investigation prompt
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The issue-0176 conformance gate (`firstUnimplementedMethod` in
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`src/ir/lower/protocol.zig`) checks method PRESENCE (and rejects `type_params >
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0`), but does NOT check that the impl method's SIGNATURE (parameter types,
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arity, return type) matches the protocol method's declared signature. A
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mismatched-type impl builds a thunk that calls the impl with the wrong ABI,
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silently miscompiling. Add signature validation when registering / gating an
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impl method against its protocol method: compare the impl method's params
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(after the erased `self`) and return type against the protocol declaration, and
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emit a located diagnostic on mismatch (arity, param type, or return type). The
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protocol method declaration is in `protocol_decl_map`; the impl FnDecl is in
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`fn_ast_map`. Decide whether this lives in the conformance gate or in
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`ProtocolResolver.registerImplBlock` (`src/ir/protocols.zig`). Follow the
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no-silent-fallback rule. Verify: the repro is now a clean diagnostic (exit 1);
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a correctly-typed impl still works; add an `examples/diagnostics/11xx-...`
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negative regression. (Found during adversarial review of issue 0176.)
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