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fix: diagnose non-conforming protocol erasure instead of unreachable-thunk SIGABRT (issue 0176)

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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).
This commit is contained in:
agra
2026-06-23 02:13:30 +03:00
parent 3605165398
commit 3c738695dc
16 changed files with 268 additions and 3 deletions
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20
examples/diagnostics/1197-diagnostics-protocol-erasure-no-impl.sx Normal file
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// Erasing a concrete type to a protocol it does NOT `impl`-ement is a hard
// error, not a silent SIGABRT. Regression (issue 0176): a plain free function
// `speak :: (self: *Dog)` with a matching receiver does NOT satisfy the
// protocol — protocol erasure is impl-driven, not structural (specs.md
// §"Storage and protocol conformance"). Before the fix, erasure built a vtable
// of unreachable thunks and `h.s.speak()` aborted with exit 133 and no output;
// now it reports a clear diagnostic at the erasure site and exits 1.
#import "modules/std.sx";
Speaker :: protocol { speak :: (self: *Self) -> i64; }
Dog :: struct { n: i64 = 0; }
speak :: (self: *Dog) -> i64 { return self.n; } // free fn — NOT an impl
Holder :: struct { s: Speaker; b: i64 = 0; }
main :: () {
d := Dog.{ n = 42 };
h : Holder = .{ s = d, b = 5 }; // <- 'Dog' does not implement 'Speaker'
print("{}\n", h.s.speak());
}

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examples/diagnostics/1198-diagnostics-protocol-erasure-generic-method.sx Normal file
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// Erasing a concrete type to a protocol whose `impl` method introduces its OWN
// type parameter is a hard error, not a silent SIGABRT. Regression (issue 0176,
// adversarial-review gap 1): `impl Speaker for Dog { speak :: (self: *Dog, $T:
// Type) }` has a matching qualified name `Dog.speak`, so the conformance gate
// used to ACCEPT it — but the thunk's `lazyLowerFunction("Dog.speak")` bails on
// `type_params.len > 0` (generics are monomorphized, not registered), so
// `resolveFuncByName` returns null and the thunk hit its `else => unreachable`
// arm: a SILENT SIGABRT (exit 133, no output) at the first dispatch. The gate
// now mirrors the thunk exactly — a method that introduces its own type params
// is a SIGNATURE MISMATCH, reported here at the erasure site (exit 1).
#import "modules/std.sx";
Speaker :: protocol { speak :: (self: *Self) -> i64; }
Dog :: struct { n: i64 = 0; }
impl Speaker for Dog { speak :: (self: *Dog, $T: Type) -> i64 { return self.n; } }
main :: () {
d := Dog.{ n = 42 };
s : Speaker = d; // <- 'Dog.speak' has a mismatched signature
print("{}\n", s.speak());
}

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examples/diagnostics/expected/1197-diagnostics-protocol-erasure-no-impl.exit Normal file
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1

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examples/diagnostics/expected/1197-diagnostics-protocol-erasure-no-impl.stderr Normal file
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error: 'Dog' does not implement protocol 'Speaker': no `impl Speaker for Dog` provides method 'speak' (protocol erasure is impl-driven — a plain or `ufcs` free function with a matching receiver does not satisfy a protocol)
--> examples/diagnostics/1197-diagnostics-protocol-erasure-no-impl.sx:18:16
|
18 | h : Holder = .{ s = d, b = 5 }; // <- 'Dog' does not implement 'Speaker'
| ^^^^^^^^^^^^^^^^^

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examples/diagnostics/expected/1198-diagnostics-protocol-erasure-generic-method.exit Normal file
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1

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examples/diagnostics/expected/1198-diagnostics-protocol-erasure-generic-method.stderr Normal file
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error: 'Dog' does not implement protocol 'Speaker': method 'speak' has a mismatched signature — a protocol-method impl must not introduce its own type parameters (e.g. `$T: Type`); it must match the protocol's signature exactly
--> examples/diagnostics/1198-diagnostics-protocol-erasure-generic-method.sx:20:3
|
20 | s : Speaker = d; // <- 'Dog.speak' has a mismatched signature
| ^^^^^^^^^^^^^^^^

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