3e8d003e3d
lowerIfExpr emitted optional_has_value only for the binding form; a bare
'if opt' passed the raw {T,i1} aggregate to condBr, where emitCondBr's
catch-all struct arm silently folded it to 'i1 true' (structs always
truthy) — a silent miscompile that took the present-branch for null
optionals. while / and / or shared the same defect.
Reduce bindingless optional conditions to optional_has_value in
lowerIfExpr/lowerWhile and via a new lowerBoolCondition helper for and/or
operands. Replace the silent-true emitCondBr arm with a lowering-time
diagnostic (checkConditionType/isValidConditionType) rejecting conditions
whose type isn't bool/integer/pointer/optional; the backend @panic is now
an unreachable tripwire.
Regressions: examples/optionals/0908..0910 + diagnostics/1194 (negative).
Verified by 3+3 adversarial reviews.
Filed adjacent bugs found during review: 0168 (array-of-optionals element
load), 0169 (optional->bool coercion), 0170 (closure-optional layout).
72 lines
3.4 KiB
Markdown
72 lines
3.4 KiB
Markdown
# 0164 — `if <optional>` with no binding silently folds the has_value test to `true`
|
||
|
||
> **RESOLVED.** Two colluding sites: `lowerIfExpr` emitted `optional_has_value`
|
||
> only for the binding form, and `emitCondBr`'s catch-all struct arm silently
|
||
> folded any non-i1 condition to `i1 true` ("structs always truthy"). Fix: reduce
|
||
> a bindingless optional condition to `optional_has_value` in `lowerIfExpr`/
|
||
> `lowerWhile`, add a shared `lowerBoolCondition` helper for `and`/`or` operands
|
||
> (the same defect affected `while`/`and`/`or`), and add a lowering-time
|
||
> diagnostic (`checkConditionType`/`isValidConditionType` in `lower/expr.zig`)
|
||
> rejecting conditions whose type isn't bool/integer/pointer/optional — turning
|
||
> the `emitCondBr` silent-true into a real type error and leaving the backend
|
||
> `@panic` as an unreachable tripwire. Regressions:
|
||
> `examples/optionals/0908-if-optional-no-binding.sx`,
|
||
> `0909-optionals-while-no-binding.sx`,
|
||
> `0910-optionals-and-or-optional-operands.sx`,
|
||
> `examples/diagnostics/1194-diagnostics-condition-non-bool-type.sx` (negative).
|
||
> Verified by 3 + 3 adversarial reviews. (Adjacent bugs found during review and
|
||
> filed separately: 0168 array-of-optionals element load, 0169 optional→bool
|
||
> coercion, 0170 closure-optional layout.)
|
||
|
||
## Symptom
|
||
|
||
Branching on an optional **without a binding** (`if opt { ... }`) takes the
|
||
present-branch unconditionally for any optional whose LLVM representation is a
|
||
struct (`?i64`, `?T`, `?f64`, …). The has_value flag is never read — the IR
|
||
emits `br i1 true`. SILENT MISCOMPILE (no diagnostic, wrong runtime result).
|
||
|
||
Pointer-sentinel optionals (`?cstring`, `?*T`, `?Closure`) are unaffected — they
|
||
lower to a bare `ptr` and hit the correct `icmp` path. The `if opt |x| { ... }`
|
||
*binding* form is also correct (it emits `optional_has_value`).
|
||
|
||
Observed vs expected: the repro prints `present` for a null optional; expected
|
||
`absent`.
|
||
|
||
## Reproduction
|
||
|
||
```sx
|
||
#import "modules/std.sx";
|
||
check :: (n: ?i64) { if n { print("present\n"); } else { print("absent\n"); } }
|
||
main :: () {
|
||
a : ?i64 = null;
|
||
b : ?i64 = 42;
|
||
check(a); // prints "present" — WRONG, expected "absent"
|
||
check(b); // prints "present" — correct
|
||
}
|
||
```
|
||
|
||
Reproduces identically for function-return init, literal-`null` init,
|
||
literal-value init, and param-passed optionals — universal, not init-path
|
||
specific.
|
||
|
||
## Investigation prompt
|
||
|
||
Two sites collude:
|
||
|
||
- `src/ir/lower/control_flow.zig` (`lowerIfExpr`, ~lines 69–72) emits
|
||
`optional_has_value` **only when `ie.binding_name != null`**. For a bindingless
|
||
`if opt`, it passes `cond = opt_val` (the raw `{T,i1}` aggregate) straight to
|
||
`condBr`. Fix: emit `optional_has_value` whenever the condition's resolved type
|
||
is an optional, binding or not.
|
||
- `src/backend/llvm/ops.zig` (`emitCondBr`, ~lines 2378–2383) has a catch-all
|
||
`else`/struct arm that does `cond = LLVMConstInt(i1, 1, 0)` with the comment
|
||
"Struct values are always truthy". This is exactly the REJECTED
|
||
silent-fallback pattern (see CLAUDE.md). After the lowering fix, make this arm
|
||
a LOUD bail (a non-i1, non-pointer condition reaching condBr is a compiler
|
||
bug) rather than a silent `true`.
|
||
|
||
Verify: the repro prints `absent` / `present`; check the IR no longer contains
|
||
`br i1 true` for the optional condition. Add an
|
||
`examples/optionals/09xx-if-optional-no-binding.sx` regression covering null
|
||
and present `?i64`/`?T` without a binding, both branches.
|