sx/issues/0129-negated-error-binding-bitwise-not.md

RESOLVED — 0129: if !e held on a SET error binding (logical not lowered bitwise)

RESOLVED (2026-06-12). Root cause: the .not arm of unary lowering (src/ir/lower/expr.zig) emitted bool_not — LLVM's bitwise Not — for EVERY operand type. On a real i1 bool that is logical not; on an error binding (an error-set value, a u32 tag at the LLVM level) a bitwise not of a nonzero tag is still nonzero, so the branch condition stayed truthy: if e and if !e BOTH held on a set error, and the !e branch read the uninitialized success value. Plain integers had the same hole (!7 was ~7 — truthy). Fix: ! is now truthiness-aware — bool keeps bool_not; integers and error-set values lower as the complement operand == 0 (cmp_eq against a typed zero); any other operand type is DIAGNOSED ("'!' needs a bool, integer, or error operand") instead of silently bit-flipped. Regression tests: examples/1057-errors-negated-error-binding.sx (set error: !e must not hold; success: !e holds with a real value; !7/!0 integer truthiness) and examples/1171-diagnostics-logical-not-bad-operand.sx (!"text" diagnosed); both FAIL on pre-fix master. Gates: zig build test 426/426, tests/run_examples.sh 600/600.

Symptom

if e { ... } on an error binding from a value-carrying failable correctly tests "error is set", but if !e { ... } evaluates TRUE even when the error IS set — both branches run, and the success value read in the !e branch is uninitialized garbage.

Hit in production: /Users/agra/projects/distribution tests/sqlite_api.sx (2026-06-12) — a close() behind if !ne segfaulted on a garbage handle. The interim workaround routed negated error logic through a plain bool.

Reproduction

#import "modules/std.sx";

E :: error { Boom }

f :: (fail: bool) -> (i64, !E) {
    if fail { raise error.Boom; }
    return 42;
}

main :: () -> i32 {
    v, e := f(true);
    if e { print("error set\n"); }
    if !e { print("BUG: !e true on a set error (v={})\n", v); return 1; }
    return 0;
}

Observed at master ba37d0b: prints both lines, v is garbage, exits 1. Expected: prints only "error set", exits 0.

Investigation prompt

Suspected area: the unary .not lowering in src/ir/lower/expr.zig — it emits bool_not (src/backend/llvm/ops.zig emitBoolNot → LLVMBuildNot, a bitwise xor-with-all-ones) regardless of operand type. Error bindings are error-set values backed by a u32 tag (src/backend/llvm/types.zig lowers .error_set to i32), so ~tag of a set error is nonzero and the condition holds. Fix: make ! truthiness-aware (complement-of-zero for integer-backed operands), or diagnose non-bool operands; silent wrong evaluation is the worst of both. Verify with the repro plus an integer-truthiness case, and run zig build test + tests/run_examples.sh.