sx/issues/0049-new-form-variadic-cross-module-llvm-emit-crash.md

# 0049 — new-form variadic `..name: []Type` defined in an imported module crashes LLVM emit

## Symptom

A pack-fn declared with the **new** variadic syntax
`..name: []Type` (the form the FFI plan migrates to, replacing
the legacy `name: ..Type`) crashes LLVM IR emission with a
null-operand `LLVMBuildExtractValue` inside `emitStrCmp` when:

1. The pack-fn lives in an **imported** module (e.g. `library/modules/std.sx`).
2. The caller is in a **different** module than the definition.

The same function written with the legacy `name: ..Type` syntax
compiles and runs cleanly. The same new-form definition placed
**locally** in the caller's module (or shadowing the imported
name) also compiles cleanly. The two together — new form + import
boundary — are what trip the emit.

```
Segmentation fault at address 0x0
???:?:?: 0x... in __ZN4llvm5Value11setNameImplERKNS_5TwineE (.../libLLVM.dylib)
???:?:?: 0x... in _LLVMBuildExtractValue (.../libLLVM.dylib)
/Users/agra/projects/sx/src/ir/emit_llvm.zig:3570:48: in emitStrCmp (sx)
        const rhs_ptr = c.LLVMBuildExtractValue(b, rhs, 0, "str.rp");
                                                ^
/Users/agra/projects/sx/src/ir/emit_llvm.zig:1805:45: in emitInst (sx)
            .str_eq => |bin| self.emitStrCmp(bin, true),
```

So an emitted `.str_eq` op has a `rhs` Ref that resolves to a null
LLVM Value. The `.str_eq` is somewhere downstream of the
new-form pack-fn's monomorphisation — most likely an
`any_to_string` / format-side string comparison that the migrated
call path threads back. The `rhs` was either never materialised in
the imported-mono's IR, or was registered against a stale
function/module slot that `emit_llvm` resolves to null.

## Reproduction

Modify `library/modules/std.sx`:

```sx
- path_join :: (parts: ..string) -> string {
+ path_join :: (..parts: []string) -> string {
```

Body unchanged. Then:

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

main :: () {
    p := path_join("a", "b");
    print("{}\n", p);
}
```

`zig build && ./zig-out/bin/sx run repro.sx` → segfault as above.

Negative controls (all compile and run fine):

```sx
// Same body, OLD form — works:
path_join :: (parts: ..string) -> string { ... }

// New form but defined LOCALLY in the caller:
path_join :: (..parts: []string) -> string { ... }
#import "modules/std.sx";  // imported anyway, just no path_join call
main :: () { p := path_join("a", "b"); ... }

// New form with `$`-prefixed pack name — works EITHER locally or imported:
path_join :: (..$parts: []string) -> string { ... }
```

So the bug is specifically:
- new-form variadic (`..name: []Type`)
- WITHOUT the `$` prefix on `name`
- defined in a module that gets imported (not in the caller's own file)

Suite state when the bug first surfaced: commit `0ede097` (master,
2026-05-27, just after the issue-0048 fix landed and the suite
was green at 213/213). The only delta on top is the
`path_join :: (parts: ..string)` → `path_join :: (..parts: []string)`
edit in `library/modules/std.sx`.

## Investigation prompt

The FFI plan migrates all stdlib variadic decls from the legacy
form to the new `..name: []Type` form (`path_join`, `format`,
`print`, plus the extern `open` decl, plus the example fixtures).
Per the FFI cadence rule the migration is supposed to be a
mechanical textual change with identical semantics. This bug
blocks that.

The fault location (`emitStrCmp` line 3570 with null `rhs`) is the
crash point, not the root cause. The root cause is one of:

1. **Cross-module pack-fn monomorphisation** — the new-form path
   in `monomorphizePackFn` registers the mono'd function in the
   current module, but if the mono'd body uses a Ref that
   resolves through a stale module/function context, the LLVM
   pass through `emit_llvm.functions[fid]` lookup hands back a
   null. Compare the new-form mono path to the legacy `name:
   ..Type` mono path side-by-side — look for any place the latter
   threads the caller's module ID / FuncId but the former forgets
   to.
2. **Synthesised slot names** — the new-form pack-fn body uses
   synthesised `__pack_<name>_<i>` idents for per-position arg
   substitution (per CHECKPOINT-FFI step-2b). If these are
   re-emitted in the caller's IR against the imported function's
   body without re-resolving against the caller's scope, they'd
   appear as undef in the final LLVM pass.
3. **The `$` workaround as a hint** — the bug disappears when the
   pack name is `..$parts: []string`. Inside the parser /
   `isPackFn` discriminator, the `$` prefix routes the function
   through the heterogeneous-pack mono path; the new-form
   WITHOUT `$` likely routes through a near-but-not-identical
   path. Diff the two routes — what does the `$` version do that
   the no-`$` version skips when the call crosses an import?

Where to start:

- `src/ir/lower.zig` — `monomorphizePackFn` (around line 8460),
  `materialisePackSlice` (around line 8261), `buildPackSliceValue`
  (around line 8225). Trace which gets called for the new-form
  no-`$` path.
- `src/ir/lower.zig:lowerPackFnCall` — call-site mono dispatch.
  Look for a `$`-prefix branch.
- `src/parser.zig:parseParam` (variadic handling) — confirm what
  AST shape the two forms produce. The new form sets `is_variadic
  = true` AND `is_comptime = false` (no `$`); the new form WITH
  `$` sets both true. The mono path probably gates on `is_comptime`.

Verification step:

After the fix, run with the path_join edit re-applied:

```sh
git diff library/modules/std.sx   # confirm new form lands
./zig-out/bin/sx run examples/121-ios-sim-bundle.sx
bash tests/run_examples.sh
```

Both should be green, no segfault.

A regression test goes in
`examples/NNN-new-form-variadic-cross-module.sx` once the fix
lands. The migration of `path_join`, `format`, `print`, and
`open` then proceeds.

## Why this matters

The FFI plan calls for the legacy `name: ..Type` form to be
dropped entirely (`current/CHECKPOINT-FFI.md` references the
`'args: ..Any' is in the plan to change to '..args: []Any'`
migration). Every stdlib consumer plus the chess game needs the
new form. With this bug, stdlib can't be migrated — moving any
stdlib variadic to the new form breaks every program that imports
it. The migration is gated on this fix.