sx/issues/0076-stack-struct-addrof-passed-by-value.md

0076 — builtin/reserved type name wrongly accepted as an identifier

Status: RESOLVED.

Root cause: the language accepted a value binding (local/global var or a parameter) spelled as a reserved/builtin type name. The parser turns such a spelling into a .type_expr rather than an .identifier (parser.zig, via Type.fromName), so the address-of family in src/ir/lower.zig never saw a scoped local and fell through to value lowering — loading the whole aggregate and passing it by value to a ptr parameter (LLVM verifier abort, or a silent *self-mutation-losing copy).

Fix: a declaration-site diagnostic in the existing semantic pass src/ir/semantic_diagnostics.zig (UnknownTypeChecker). New checkBindingName rejects any parameter name or var binding name (local or global, := / typed-local forms) whose spelling collides with a reserved type name; isReservedTypeName defers to the parser's own classifier (types.Type.fromName) so the rejected set never drifts from the set that would parse as a type — the named builtins (bool, string, void, f32, f64, usize, isize, Any) and [su]N over sx's 1–64 range. Bare value names (s, self, index) are untouched. No lowering special-case is added; the .identifier-only address-of paths are correct once type-shaped names can never be bound. The rejected bareVarName approach was never landed.

Regression tests:

• examples/0125-types-type-named-var-rejected.sx — := form (s2) rejected.
• examples/1119-diagnostics-reserved-type-name-as-identifier.sx — parameter (u8), typed-local (s64, bool), and := (string) forms rejected.
• examples/0135-types-self-streaming-nonreserved.sx — positive: *self streaming with non-reserved names (hasher, ctx) accumulates correctly via both update(@h, …) and h.update(…).

Pre-existing example examples/0904-... declared locals s1/s2 (incidental names); renamed to filled/empty. Scope: main-file decls only, matching the pass's existing trusted-imports convention.

Symptom (how it first surfaced)

A local variable whose name is lexically a type — e.g. s2 (the sN arbitrary-width signed-int syntax: Type.fromName("s2") → s(2)), or u8, s64, etc. — is accepted as a variable. Because such a name parses as a .type_expr (not .identifier), the address-of family of lowering sites (@s2, the autoref s2.update(...) receiver, a bare f(s2) at a *T param, global function-pointer args) does NOT recognize it as a scoped local and falls through to value lowering — loading the whole aggregate and passing it by value to a ptr parameter:

LLVM verification failed: Call parameter type does not match function signature!
  call void @update(ptr @__sx_default_context,
    { [8 x i64], [64 x i8], i64, i64 } %load, ...)

For some struct shapes it compiles but silently passes a copy (callee *self mutations lost). A non-type-shaped name (hasher, ctx) never triggers any of this — the .identifier paths already work correctly.

Root cause

The language is accepting reserved/builtin type names as identifiers in the first place. sN/uN (arbitrary-width ints) and the named builtins (bool, string, void, f32, f64, s8/s16/s32/s64, u8/u16/u32/u64, …) are reserved type names; declaring a variable with such a name is meaningless and produces the mis-lowering above. Patching each address-of site to tolerate the name (the rejected bareVarName approach) is whack-a-mole — there is always another site, and it entrenches a name that should never have been allowed.

Proper fix (the required direction)

Emit a diagnostic error when an identifier is declared with a name that collides with a builtin/reserved type name — including the arbitrary-width [su][0-9]+ (sN/uN) family AND the named builtins (bool, string, void, f32, f64, the fixed-width int types, etc.). Scope ruling (Agra): all builtin/reserved type names are rejected as identifiers. (User-defined struct/type-name shadowing, if intentionally supported elsewhere, is out of scope for this issue — this is specifically about builtin/reserved type names.)

Diagnostic at the declaration site, e.g.: error: 'u8' is a reserved type name and cannot be used as an identifier with the declaration's span.

Suspected area: name binding / declaration handling — where a := / typed local / parameter name is introduced. Reject the name there, before it ever reaches lowering. Do NOT add lowering special-cases for type-shaped names; the .identifier-only checks at the address-of sites are then correct as-is (no type-shaped name can reach them).

Reproduction

#import "modules/std.sx";
Sha256 :: struct { h:[8]u64; block:[64]u8; block_len:s64=0; total_len:u64=0; }
init :: () -> Sha256 { s:Sha256=---; s.block_len=0; s.total_len=0; s }
update :: (self:*Sha256, data:string) { self.total_len += data.len; }
main :: () -> s32 { s2 := init(); update(@s2, "."); print("total_len={}\n", s2.total_len); return 0; }

./zig-out/bin/sx run <file> today → LLVM verifier abort. Expected after fix: a clean compile-time diagnostic that s2 is a reserved type name and cannot be an identifier (exit non-zero, readable error — NOT an LLVM abort, NOT a silent copy). The same program with a non-reserved name (hasher := init(); update(@hasher, ".")) must compile and print total_len=1.

Verification

1. Pinned diagnostics test(s) asserting the error for representative reserved names used as identifiers: s2, u8, s64, bool, string (declaration forms: :=, typed local, and a parameter name). Capture the diagnostic text in expected/.
2. A positive test: the same *self streaming pattern with NON-reserved names (hasher, ctx) compiles and accumulates state correctly via both update(@h, ...) and h.update(...) — proving the .identifier paths are correct and no lowering special-case is needed.
3. zig build && zig build test && bash tests/run_examples.sh all green. If any existing example/test declares a variable with a reserved type name, it is now illegal — fix the test's variable name (do NOT weaken the diagnostic). Report how many such sites existed.

Provenance

Discovered by the distribution flow (P1.2 pure-sx SHA-256), whose minimal repro happened to name a local s2. Real SHA-256 code with names like hasher/ctx is unaffected on the current compiler — so the P1.2 "blocker" was a naming artifact, and this issue is really a missing-diagnostic correctness bug.