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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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4
examples/protocols/expected/0419-protocols-struct-field-dispatch.stdout Normal file
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@@ -0,0 +1,4 @@
lit: 52 7 15
reassign: 20 50
nested: 42 10
passed: 62