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fix: reject implicit ?T -> bool coercion instead of silent false (issue 0169)

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The Optional->Concrete unwrap classify rule treated ?i64 -> bool as
unwrap+narrow (both builtin), silently yielding false for every optional
(present or null). specs.md defines no implicit optional->bool
conversion. Reject it: conversions.zig adds an optional_to_bool_reject
plan (dst == bool, child != bool); coerce.zig emits a located diagnostic
suggesting '!= null'. Covers arg/field-init/return via the shared
coerceMode. The if-opt presence test (issue 0164) is a separate path,
untouched.

Regression: examples/diagnostics/1199-diagnostics-optional-to-bool.sx +
conversions.test.zig unit test. Verified by 3 adversarial reviews, suite
789/0. Filed adjacent issue 0179 (whole implicit ?T->concrete unwrap
family silently miscompiles a null optional; design-touching).
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2026-06-23 02:47:51 +03:00
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issues/0169-optional-to-bool-coercion-silent-false.md
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# 0169 — optional passed where `bool` is expected silently coerces to `false` (always)
> **RESOLVED** (scoped to `?T → bool`, `T ≠ bool`). The "Optional → Concrete
> unwrap" classify rule treated `?i64 → bool` as unwrap+narrow (both i64 and bool
> are builtin), silently yielding `false`. specs.md defines no implicit
> optional→bool conversion, so the fix REJECTS it: `src/ir/conversions.zig` adds
> an `optional_to_bool_reject` plan (when `dst == bool` and `child ≠ bool`);
> `src/ir/lower/coerce.zig` emits a located diagnostic ("…use '!= null'…")
> instead of a constant false. Covers arg / field-init / return (all share
> `coerceMode`). `if opt` presence-test (issue 0164) is a separate path,
> untouched. Verified by 3 adversarial reviews; suite 789/0. Regression:
> `examples/diagnostics/1199-diagnostics-optional-to-bool.sx` + a
> `conversions.test.zig` unit test. NOTE: review surfaced a larger sibling
> surface — the whole implicit `?T → concrete` unwrap family (incl. the
> `?bool → bool` cell still allowed here) silently miscompiles a NULL optional to
> garbage — filed as **0179** (design-touching: needs the flow-narrowing
> decision).
## Symptom
Passing an optional (`?T`) to a `bool` parameter (or any bool-typed position: