# 0150 — a `void` struct field crashes the compiler (unsized-type SIGTRAP in LLVM)

## Status
OPEN — surfaced by Stream B1 (fibers) B1.2: `Future(void)` (needed by
`timeout(io, ms) -> Future(void)`) instantiates a struct with a `result: void`
field, which hits this bug. Independent of the fibers work (a plain
`struct { v: void; }` reproduces it standalone).

## Symptom
Declaring or instantiating any struct that has a field of type `void` aborts the
compiler with `SIGTRAP` (exit 133/134) — no sx diagnostic. The trap is LLVM's
`llvm_unreachable("Cannot getTypeInfo() on a type that is unsized!")`:

```
libLLVM`llvm::DataLayout::getTypeSizeInBits + 912   brk #0x1   (EXC_BREAKPOINT)
```

Reached via `declareFunction` → `toLLVMType(func.ret)` when a function returns
such a struct, or directly when laying out the struct.

Observed: SIGTRAP, no output, no diagnostic.
Expected: either zero-size the `void` field (a `void`/zero-sized field is a
legitimate construct — cf. Zig) OR emit a clean type diagnostic
("a struct field may not have type `void`") — never a raw backend crash.

## Reproduction
```sx
#import "modules/std.sx";

Holder :: struct { v: void; ok: bool; }

main :: () -> i32 {
    h : Holder = .{ ok = true };
    if h.ok { print("ok\n"); }
    return 0;
}
```
`./zig-out/bin/sx run repro.sx` → SIGTRAP (exit 133), no output.

Also reproduces through a generic: `Box :: struct($T: Type) { v: T; }` then
`Box(void)` — i.e. any monomorphization that binds a struct field to `void`.

## Suspected area
- `src/backend/llvm/types.zig` `toLLVMTypeInfo` (struct field loop ~line 111):
  a `void` field's LLVM type is the unsized `void` type, then `getTypeSizeInBits`
  on the enclosing struct traps.
- The type layout / size code (`src/ir/types.zig` `typeSizeBytes` and the LLVM
  struct builder) should treat a `void` field as zero-sized (skip it in the LLVM
  struct, size 0, align 1) — the same way a zero-field struct is handled.

## Investigation prompt (paste into a fresh session)
> A `void` struct field crashes the sx compiler with an unsized-type SIGTRAP in
> LLVM `getTypeSizeInBits` (no diagnostic). Repro: `issues/0150-...` (run it →
> exit 133). Decide the semantics: a `void` field should be ZERO-SIZED (preferred
> — it is a legitimate construct, e.g. `Future(void).result`), laid out as
> nothing (size 0, align 1) and OMITTED from the LLVM struct body; OR, if
> zero-sized fields are out of scope, a clean front-end diagnostic ("a struct
> field may not have type `void`, found in field `<name>` of `<Struct>`") before
> emission — NEVER a backend trap. Likely sites: `src/backend/llvm/types.zig`
> `toLLVMTypeInfo` (skip `void` fields when building the LLVM struct element
> list) + `src/ir/types.zig` size/align (`typeSizeBytes`/align: a `void` field
> contributes 0). If choosing the diagnostic route, add it where struct fields
> are validated at type-resolution time. Verify: the repro prints `ok` (zero-size
> route) or emits the diagnostic + clean exit 1 (diagnostic route); then move the
> repro into `examples/` as a regression test.

## Why this matters for B1 (fibers)
`Future($R)` with `$R = void` is the natural shape for `timeout(io, ms) ->
Future(void)` (B1.2 spec) and for any future-of-no-value. B1.2 deferred
`timeout` pending this fix rather than route around it with a substitute
non-void shape (which would hide the bug). Once 0150 lands, re-add `timeout`
with `Future(void)` (see the saved WIP at `.sx-tmp/b12-wip/io.sx`).
