sx/issues/0060-closure-literal-composition-miscompiles.md

# 0060 — closure-literal composition miscompiles (blocks ERR/E5.1)

> **✅ RESOLVED.** A closure's underlying function carries a hidden `env` arg
> that a bare `(T) -> U` slot doesn't pass, so a closure flowing into a bare
> function-type slot dropped the env (the first user arg landed in the env slot;
> the rest read garbage). Fixes (all in this commit):
> - **`src/parser.zig`** — `isLambda` now accepts `.bang` in the return-type
>   lookahead, so failable closure literals (`-> !` / `-> (T, !)`) parse.
> - **`src/ir/lower.zig`** — `createClosureToBareFnAdapter`: a capture-free
>   closure flowing into a bare `(T) -> U` slot is bridged by a generated adapter
>   carrying the bare ABI (forwards a null env). `lowerLambda` returns the
>   adapter `func_ref` for that case. Rejected (no silent miscompile): a
>   **capturing** closure into a bare slot (env has nowhere to live), and a
>   **failable** closure into a **non-failable** slot (the FFI-boundary rule).
> - **`src/ir/lower.zig`** — arrow-body failable closures (`-> (T, !) => expr`)
>   now wrap the bare success value into `{value, 0}` via
>   `lowerFailableSuccessReturn` (the implicit return previously coerced a bare
>   value into the failable tuple and returned `0`).
>
> Regression tests: `examples/0309-closures-literal-as-bare-fn-param.sx`
> (non-failable, block + arrow, called inside the callee) and
> `examples/1039-errors-failable-closure-literal.sx` (failable closures, block +
> arrow, direct + `Closure(...)` param).
>
> **Remaining E5.1 follow-up (not 0060):** calling a **bare** failable
> function-type param (`cb: (s64) -> (s64, !E)`) resolves the call result as
> `unresolved` (the idiomatic `Closure(s64) -> (s64, !E)` form works); the
> non-failable→failable widening adapter is currently *rejected* rather than
> generated; and the program-wide SCC union per closure shape is unimplemented.

## Symptom

A `closure(...)` literal passed **directly as a function-type argument**, where
the callee invokes it, produces wrong values. Surfaced while implementing ERR
E5.1 (composition with closures), but it is **not** error-specific — plain
non-failable closures miscompile too.

`issues/0060-closure-literal-composition-miscompiles.sx`:

```sx
#import "modules/std.sx";
apply :: (f: (s64) -> s64) -> s64 { return f(5); }
main :: () {
    print("block={}\n", apply(closure((x: s64) -> s64 { return x * 2; })));  // want 10
    print("arrow={}\n", apply(closure((x: s64) -> s64 => x * 2)));           // want 10
}
```

- **Expected:** `block=10`, `arrow=10`.
- **Actual:** `block=192`, `arrow=20` (exit 0 — silent miscompile, no diagnostic).

**Working contrast:** `examples/0302-closures-closures.sx` —
`apply :: (f, x) -> s64 { return f(x); }` called as `apply(closure(... => ...), 10)`
works. There the piped value arrives as a *separate* argument; here the callee
calls the closure param with a *literal*, and the literal/closure-env marshalling
is wrong. Likely an env/arg-slot mixup when a closure literal is materialized as a
call argument and then invoked with a constant inside the callee.

## Failable-closure follow-ons (the actual E5.1 surface)

Failable closures (`closure((x) -> (T, !) { ... })`) are the point of E5.1.
Two further gaps sit on top of 0060:

1. **Parser — `isLambda` doesn't accept a `!` return type.** A closure/lambda
   literal with `-> !` / `-> (T, !)` fails to parse ("expected ','") because the
   return-type token-skipper in `isLambda` (`src/parser.zig`, the `arrow` branch
   ~line 3302) omits `.bang`. One-line fix:

   ```zig
   //   self.current.tag == .star or self.current.tag == .question)
   // becomes:
       self.current.tag == .star or self.current.tag == .question or
       self.current.tag == .bang)
   ```

   With that patch, failable closure literals parse and **block-body, directly-
   called** ones work end-to-end (success / `catch` / `or` all correct).

2. **Arrow-body failable closures miscompile.** After the parser patch,
   `n := closure((x: s64) -> (s64, !E) => x + 1); n(40) catch e 0` returns `0`
   instead of `41` — the value slot reads as undef/0. Block-body equivalents
   are correct, so it's an arrow-body (`=>`) failable-closure lowering bug
   (the expression-body return isn't assembled into the `{value, error}` tuple
   the same way the block-body path does). Compare `lowerLambda`
   (`src/ir/lower.zig` ~7617) block vs arrow return handling against the named-
   function failable return path (`lowerFailableSuccessReturn`).

## Investigation prompt (paste into a fresh session)

> Closure literals passed as a function-type argument miscompile when the callee
> calls them: `apply :: (f: (s64)->s64) -> s64 { return f(5); }` then
> `apply(closure((x: s64) -> s64 { return x*2; }))` prints 192 (want 10); the
> arrow form prints 20. The working pattern (examples/0302) passes the value as a
> separate arg. Suspect the closure-literal-as-call-argument lowering: the
> closure env / the inner call's constant argument is marshalled into the wrong
> slot. Look at how a `closure(...)` literal in argument position is lowered
> (closure construction + the `Closure` calling convention) in `src/ir/lower.zig`
> / `src/ir/emit_llvm.zig`, vs the working separate-arg path.
>
> Then unblock ERR/E5.1: (a) apply the one-line `isLambda` `.bang` patch above;
> (b) fix arrow-body failable closure lowering (returns 0). Verify with a new
> `examples/XXXX-errors-failable-closure-literal.sx`: a block-body and an
> arrow-body failable closure, called directly, consumed by `catch` / `or`; and a
> failable closure passed as a `(T)->(U,!)` parameter and `try`-called inside the
> callee.

## Impact

Blocks ERR/E5.1 (composition with closures/methods/generics): every E5.1
sub-feature — failable closures as parameters, the program-wide SCC union per
closure shape, the FFI rejection check, and the non-failable→failable widening
adapter — needs closure literals to compose correctly first. Per the project's
impassable rule, E5.1 is paused here rather than built on miscompiling closures.